6. Protocols
6.1 Babel
Introduction
The Babel protocol (RFC 6126) is a loop-avoiding distance-vector routing protocol that is robust and efficient both in ordinary wired networks and in wireless mesh networks. Babel is conceptually very simple in its operation and "just works" in its default configuration, though some configuration is possible and in some cases desirable.
While the Babel protocol is dual stack (i.e., can carry both IPv4 and IPv6 routes over the same IPv6 transport), BIRD presently implements only the IPv6 subset of the protocol. No Babel extensions are implemented, but the BIRD implementation can coexist with implementations using the extensions (and will just ignore extension messages).
The Babel protocol implementation in BIRD is currently in alpha stage.
Configuration
Babel supports no global configuration options apart from those common to all other protocols, but supports the following per-interface configuration options:
protocol babel [<name>] {
interface <interface pattern> {
type <wired|wireless>;
rxcost <number>;
hello interval <number>;
update interval <number>;
port <number>;
tx class|dscp <number>;
tx priority <number>;
rx buffer <number>;
tx length <number>;
check link <switch>;
};
}
type wired|wirelessThis option specifies the interface type: Wired or wireless. Wired interfaces are considered more reliable, and so the default hello interval is higher, and a neighbour is considered unreachable after only a small number of "hello" packets are lost. On wireless interfaces, hello packets are sent more often, and the ETX link quality estimation technique is used to compute the metrics of routes discovered over this interface. This technique will gradually degrade the metric of routes when packets are lost rather than the more binary up/down mechanism of wired type links. Default:
wired.rxcost numThis specifies the RX cost of the interface. The route metrics will be computed from this value with a mechanism determined by the interface
type. Default: 96 for wired interfaces, 256 for wireless.hello interval numInterval at which periodic "hello" messages are sent on this interface, in seconds. Default: 4 seconds.
update interval numInterval at which periodic (full) updates are sent. Default: 4 times the hello interval.
port numberThis option selects an UDP port to operate on. The default is to operate on port 6696 as specified in the Babel RFC.
tx class|dscp|priority numberThese options specify the ToS/DiffServ/Traffic class/Priority of the outgoing Babel packets. See tx class common option for detailed description.
rx buffer numberThis option specifies the size of buffers used for packet processing. The buffer size should be bigger than maximal size of received packets. The default value is the interface MTU, and the value will be clamped to a minimum of 512 bytes + IP packet overhead.
tx length numberThis option specifies the maximum length of generated Babel packets. To avoid IP fragmentation, it should not exceed the interface MTU value. The default value is the interface MTU value, and the value will be clamped to a minimum of 512 bytes + IP packet overhead.
check link switchIf set, the hardware link state (as reported by OS) is taken into consideration. When the link disappears (e.g. an ethernet cable is unplugged), neighbors are immediately considered unreachable and all routes received from them are withdrawn. It is possible that some hardware drivers or platforms do not implement this feature. Default: yes.
Attributes
Babel defines just one attribute: the internal babel metric of the route. It
is exposed as the babel_metric attribute and has range from 1 to infinity
(65535).
Example
protocol babel {
interface "eth*" {
type wired;
};
interface "wlan0", "wlan1" {
type wireless;
hello interval 1;
rxcost 512;
};
interface "tap0";
# This matches the default of babeld: redistribute all addresses
# configured on local interfaces, plus re-distribute all routes received
# from other babel peers.
export where (source = RTS_DEVICE) || (source = RTS_BABEL);
}
6.2 BFD
Introduction
Bidirectional Forwarding Detection (BFD) is not a routing protocol itself, it is an independent tool providing liveness and failure detection. Routing protocols like OSPF and BGP use integrated periodic "hello" messages to monitor liveness of neighbors, but detection times of these mechanisms are high (e.g. 40 seconds by default in OSPF, could be set down to several seconds). BFD offers universal, fast and low-overhead mechanism for failure detection, which could be attached to any routing protocol in an advisory role.
BFD consists of mostly independent BFD sessions. Each session monitors an unicast bidirectional path between two BFD-enabled routers. This is done by periodically sending control packets in both directions. BFD does not handle neighbor discovery, BFD sessions are created on demand by request of other protocols (like OSPF or BGP), which supply appropriate information like IP addresses and associated interfaces. When a session changes its state, these protocols are notified and act accordingly (e.g. break an OSPF adjacency when the BFD session went down).
BIRD implements basic BFD behavior as defined in RFC 5880 (some advanced features like the echo mode or authentication are not implemented), IP transport for BFD as defined in RFC 5881 and RFC 5883 and interaction with client protocols as defined in RFC 5882.
Note that BFD implementation in BIRD is currently a new feature in development, expect some rough edges and possible UI and configuration changes in the future. Also note that we currently support at most one protocol instance.
BFD packets are sent with a dynamic source port number. Linux systems use by
default a bit different dynamic port range than the IANA approved one
(49152-65535). If you experience problems with compatibility, please adjust
/proc/sys/net/ipv4/ip_local_port_range.
Configuration
BFD configuration consists mainly of multiple definitions of interfaces.
Most BFD config options are session specific. When a new session is requested
and dynamically created, it is configured from one of these definitions. For
sessions to directly connected neighbors, interface definitions are chosen
based on the interface associated with the session, while multihop
definition is used for multihop sessions. If no definition is relevant, the
session is just created with the default configuration. Therefore, an empty BFD
configuration is often sufficient.
Note that to use BFD for other protocols like OSPF or BGP, these protocols
also have to be configured to request BFD sessions, usually by bfd option.
A BFD instance not associated with any VRF handles session requests from all
other protocols, even ones associated with a VRF. Such setup would work for
single-hop BFD sessions if net.ipv4.udp_l3mdev_accept sysctl is enabled,
but does not currently work for multihop sessions. Another approach is to
configure multiple BFD instances, one for each VRF (including the default VRF).
Each BFD instance associated with a VRF (regular or default) only handles
session requests from protocols in the same VRF.
Some of BFD session options require time value, which has to be specified
with the appropriate unit: num s|ms|us. Although microseconds
are allowed as units, practical minimum values are usually in order of tens of
milliseconds.
protocol bfd [<name>] {
interface <interface pattern> {
interval <time>;
min rx interval <time>;
min tx interval <time>;
idle tx interval <time>;
multiplier <num>;
passive <switch>;
authentication none;
authentication simple;
authentication [meticulous] keyed md5|sha1;
password "<text>";
password "<text>" {
id <num>;
generate from "<date>";
generate to "<date>";
accept from "<date>";
accept to "<date>";
from "<date>";
to "<date>";
};
};
multihop {
interval <time>;
min rx interval <time>;
min tx interval <time>;
idle tx interval <time>;
multiplier <num>;
passive <switch>;
};
neighbor <ip> [dev "<interface>"] [local <ip>] [multihop <switch>];
}
interface pattern [, ...] { options }Interface definitions allow to specify options for sessions associated with such interfaces and also may contain interface specific options. See interface common option for a detailed description of interface patterns. Note that contrary to the behavior of
interfacedefinitions of other protocols, BFD protocol would accept sessions (in default configuration) even on interfaces not covered by such definitions.multihop { options }Multihop definitions allow to specify options for multihop BFD sessions, in the same manner as
interfacedefinitions are used for directly connected sessions. Currently only one such definition (for all multihop sessions) could be used.neighbor ip [dev "interface"] [local ip] [multihop switch]BFD sessions are usually created on demand as requested by other protocols (like OSPF or BGP). This option allows to explicitly add a BFD session to the specified neighbor regardless of such requests.
The session is identified by the IP address of the neighbor, with optional specification of used interface and local IP. By default the neighbor must be directly connected, unless the session is configured as multihop. Note that local IP must be specified for multihop sessions.
Session specific options (part of interface and multihop definitions):
interval timeBFD ensures availability of the forwarding path associated with the session by periodically sending BFD control packets in both directions. The rate of such packets is controlled by two options,
min rx intervalandmin tx interval(see below). This option is just a shorthand to set both of these options together.min rx interval timeThis option specifies the minimum RX interval, which is announced to the neighbor and used there to limit the neighbor's rate of generated BFD control packets. Default: 10 ms.
min tx interval timeThis option specifies the desired TX interval, which controls the rate of generated BFD control packets (together with
min rx intervalannounced by the neighbor). Note that this value is used only if the BFD session is up, otherwise the value ofidle tx intervalis used instead. Default: 100 ms.idle tx interval timeIn order to limit unnecessary traffic in cases where a neighbor is not available or not running BFD, the rate of generated BFD control packets is lower when the BFD session is not up. This option specifies the desired TX interval in such cases instead of
min tx interval. Default: 1 s.multiplier numFailure detection time for BFD sessions is based on established rate of BFD control packets (
min rx/tx interval) multiplied by this multiplier, which is essentially (ignoring jitter) a number of missed packets after which the session is declared down. Note that rates and multipliers could be different in each direction of a BFD session. Default: 5.passive switchGenerally, both BFD session endpoints try to establish the session by sending control packets to the other side. This option allows to enable passive mode, which means that the router does not send BFD packets until it has received one from the other side. Default: disabled.
authentication noneNo passwords are sent in BFD packets. This is the default value.
authentication simpleEvery packet carries 16 bytes of password. Received packets lacking this password are ignored. This authentication mechanism is very weak.
authentication [meticulous] keyed md5|sha1An authentication code is appended to each packet. The cryptographic algorithm is keyed MD5 or keyed SHA-1. Note that the algorithm is common for all keys (on one interface), in contrast to OSPF or RIP, where it is a per-key option. Passwords (keys) are not sent open via network.
The
meticulousvariant means that cryptographic sequence numbers are increased for each sent packet, while in the basic variant they are increased about once per second. Generally, themeticulousvariant offers better resistance to replay attacks but may require more computation.password "text"Specifies a password used for authentication. See password common option for detailed description. Note that password option
algorithmis not available in BFD protocol. The algorithm is selected byauthenticationoption for all passwords.
Example
protocol bfd {
interface "eth*" {
min rx interval 20 ms;
min tx interval 50 ms;
idle tx interval 300 ms;
};
interface "gre*" {
interval 200 ms;
multiplier 10;
passive;
};
multihop {
interval 200 ms;
multiplier 10;
};
neighbor 192.168.1.10;
neighbor 192.168.2.2 dev "eth2";
neighbor 192.168.10.1 local 192.168.1.1 multihop;
}
6.3 BGP
The Border Gateway Protocol is the routing protocol used for backbone level routing in the today's Internet. Contrary to other protocols, its convergence does not rely on all routers following the same rules for route selection, making it possible to implement any routing policy at any router in the network, the only restriction being that if a router advertises a route, it must accept and forward packets according to it.
BGP works in terms of autonomous systems (often abbreviated as AS). Each AS is a part of the network with common management and common routing policy. It is identified by a unique 16-bit number (ASN). Routers within each AS usually exchange AS-internal routing information with each other using an interior gateway protocol (IGP, such as OSPF or RIP). Boundary routers at the border of the AS communicate global (inter-AS) network reachability information with their neighbors in the neighboring AS'es via exterior BGP (eBGP) and redistribute received information to other routers in the AS via interior BGP (iBGP).
Each BGP router sends to its neighbors updates of the parts of its routing table it wishes to export along with complete path information (a list of AS'es the packet will travel through if it uses the particular route) in order to avoid routing loops.
BIRD supports all requirements of the BGP4 standard as defined in RFC 4271 It also supports the community attributes (RFC 1997), capability negotiation (RFC 5492), MD5 password authentication (RFC 2385), extended communities (RFC 4360), route reflectors (RFC 4456), graceful restart (RFC 4724), multiprotocol extensions (RFC 4760), 4B AS numbers (RFC 4893), and 4B AS numbers in extended communities (RFC 5668).
For IPv6, it uses the standard multiprotocol extensions defined in RFC 4760 and applied to IPv6 according to RFC 2545.
Route selection rules
BGP doesn't have any simple metric, so the rules for selection of an optimal route among multiple BGP routes with the same preference are a bit more complex and they are implemented according to the following algorithm. It starts the first rule, if there are more "best" routes, then it uses the second rule to choose among them and so on.
- Prefer route with the highest Local Preference attribute.
- Prefer route with the shortest AS path.
- Prefer IGP origin over EGP and EGP origin over incomplete.
- Prefer the lowest value of the Multiple Exit Discriminator.
- Prefer routes received via eBGP over ones received via iBGP.
- Prefer routes with lower internal distance to a boundary router.
- Prefer the route with the lowest value of router ID of the advertising router.
IGP routing table
BGP is mainly concerned with global network reachability and with routes to other autonomous systems. When such routes are redistributed to routers in the AS via BGP, they contain IP addresses of a boundary routers (in route attribute NEXT_HOP). BGP depends on existing IGP routing table with AS-internal routes to determine immediate next hops for routes and to know their internal distances to boundary routers for the purpose of BGP route selection. In BIRD, there is usually one routing table used for both IGP routes and BGP routes.
Configuration
Each instance of the BGP corresponds to one neighboring router. This allows to set routing policy and all the other parameters differently for each neighbor using the following configuration parameters:
local [ip] as numberDefine which AS we are part of. (Note that contrary to other IP routers, BIRD is able to act as a router located in multiple AS'es simultaneously, but in such cases you need to tweak the BGP paths manually in the filters to get consistent behavior.) Optional
ipargument specifies a source address, equivalent to thesource addressoption (see below). This parameter is mandatory.neighbor [ip] [port number] [as number]Define neighboring router this instance will be talking to and what AS it is located in. In case the neighbor is in the same AS as we are, we automatically switch to iBGP. Optionally, the remote port may also be specified. The parameter may be used multiple times with different sub-options (e.g., both
neighbor 10.0.0.1 as 65000;andneighbor 10.0.0.1; neighbor as 65000;are valid). This parameter is mandatory.interface stringDefine interface we should use for link-local BGP IPv6 sessions. Interface can also be specified as a part of
neighbor address(e.g.,neighbor fe80::1234%eth0 as 65000;). The option may also be used for non link-local sessions when it is necessary to explicitly specify an interface, but only for direct (not multihop) sessions.directSpecify that the neighbor is directly connected. The IP address of the neighbor must be from a directly reachable IP range (i.e. associated with one of your router's interfaces), otherwise the BGP session wouldn't start but it would wait for such interface to appear. The alternative is the
multihopoption. Default: enabled for eBGP.multihop [number]Configure multihop BGP session to a neighbor that isn't directly connected. Accurately, this option should be used if the configured neighbor IP address does not match with any local network subnets. Such IP address have to be reachable through system routing table. The alternative is the
directoption. For multihop BGP it is recommended to explicitly configure the source address to have it stable. Optionalnumberargument can be used to specify the number of hops (used for TTL). Note that the number of networks (edges) in a path is counted; i.e., if two BGP speakers are separated by one router, the number of hops is 2. Default: enabled for iBGP.source address ipDefine local address we should use for next hop calculation and as a source address for the BGP session. Default: the address of the local end of the interface our neighbor is connected to.
next hop selfAvoid calculation of the Next Hop attribute and always advertise our own source address as a next hop. This needs to be used only occasionally to circumvent misconfigurations of other routers. Default: disabled.
next hop keepForward the received Next Hop attribute even in situations where the local address should be used instead, like when the route is sent to an interface with a different subnet. Default: disabled.
missing lladdr self|drop|ignoreNext Hop attribute in BGP-IPv6 sometimes contains just the global IPv6 address, but sometimes it has to contain both global and link-local IPv6 addresses. This option specifies what to do if BIRD have to send both addresses but does not know link-local address. This situation might happen when routes from other protocols are exported to BGP, or when improper updates are received from BGP peers.
selfmeans that BIRD advertises its own local address instead.dropmeans that BIRD skips that prefixes and logs error.ignoremeans that BIRD ignores the problem and sends just the global address (and therefore forms improper BGP update). Default:self, unless BIRD is configured as a route server (optionrs client), in that case default isignore, because route servers usually do not forward packets themselves.gateway direct|recursiveFor received routes, their
gw(immediate next hop) attribute is computed from receivedbgp_next_hopattribute. This option specifies how it is computed. Direct mode means that the IP address frombgp_next_hopis used if it is directly reachable, otherwise the neighbor IP address is used. Recursive mode means that the gateway is computed by an IGP routing table lookup for the IP address frombgp_next_hop. Note that there is just one level of indirection in recursive mode - the route obtained by the lookup must not be recursive itself, to prevent mutually recursive routes.Recursive mode is the behavior specified by the BGP standard. Direct mode is simpler, does not require any routes in a routing table, and was used in older versions of BIRD, but does not handle well nontrivial iBGP setups and multihop. Recursive mode is incompatible with sorted tables. Default:
directfor direct sessions,recursivefor multihop sessions.igp table nameSpecifies a table that is used as an IGP routing table. Default: the same as the table BGP is connected to.
check link switchBGP could use hardware link state into consideration. If enabled, BIRD tracks the link state of the associated interface and when link disappears (e.g. an ethernet cable is unplugged), the BGP session is immediately shut down. Note that this option cannot be used with multihop BGP. Default: disabled.
bfd switch|gracefulBGP could use BFD protocol as an advisory mechanism for neighbor liveness and failure detection. If enabled, BIRD setups a BFD session for the BGP neighbor and tracks its liveness by it. This has an advantage of an order of magnitude lower detection times in case of failure. When a neighbor failure is detected, the BGP session is restarted. Optionally, it can be configured (by
gracefulargument) to trigger graceful restart instead of regular restart. Note that BFD protocol also has to be configured, see BFD section for details. Default: disabled.ttl security switchUse GTSM (RFC 5082 - the generalized TTL security mechanism). GTSM protects against spoofed packets by ignoring received packets with a smaller than expected TTL. To work properly, GTSM have to be enabled on both sides of a BGP session. If both
ttl securityandmultihopoptions are enabled,multihopoption should specify proper hop value to compute expected TTL. Kernel support required: Linux: 2.6.34+ (IPv4), 2.6.35+ (IPv6), BSD: since long ago, IPv4 only. Note that full (ICMP protection, for example) RFC 5082 support is provided by Linux only. Default: disabled.password stringUse this password for MD5 authentication of BGP sessions (RFC 2385). When used on BSD systems, see also
setkeyoption below. Default: no authentication.setkey switchOn BSD systems, keys for TCP MD5 authentication are stored in the global SA/SP database, which can be accessed by external utilities (e.g. setkey(8)). BIRD configures security associations in the SA/SP database automatically based on
passwordoptions (see above), this option allows to disable automatic updates by BIRD when manual configuration by external utilities is preferred. Note that automatic SA/SP database updates are currently implemented only for FreeBSD. Passwords have to be set manually by an external utility on NetBSD and OpenBSD. Default: enabled (ignored on non-FreeBSD).passive switchStandard BGP behavior is both initiating outgoing connections and accepting incoming connections. In passive mode, outgoing connections are not initiated. Default: off.
rr clientBe a route reflector and treat the neighbor as a route reflection client. Default: disabled.
rr cluster id IPv4 addressRoute reflectors use cluster id to avoid route reflection loops. When there is one route reflector in a cluster it usually uses its router id as a cluster id, but when there are more route reflectors in a cluster, these need to be configured (using this option) to use a common cluster id. Clients in a cluster need not know their cluster id and this option is not allowed for them. Default: the same as router id.
rs clientBe a route server and treat the neighbor as a route server client. A route server is used as a replacement for full mesh EBGP routing in Internet exchange points in a similar way to route reflectors used in IBGP routing. BIRD does not implement obsoleted RFC 1863, but uses ad-hoc implementation, which behaves like plain EBGP but reduces modifications to advertised route attributes to be transparent (for example does not prepend its AS number to AS PATH attribute and keeps MED attribute). Default: disabled.
secondary switchUsually, if an export filter rejects a selected route, no other route is propagated for that network. This option allows to try the next route in order until one that is accepted is found or all routes for that network are rejected. This can be used for route servers that need to propagate different tables to each client but do not want to have these tables explicitly (to conserve memory). This option requires that the connected routing table is sorted. Default: off.
add paths switch|rx|txStandard BGP can propagate only one path (route) per destination network (usually the selected one). This option controls the add-path protocol extension, which allows to advertise any number of paths to a destination. Note that to be active, add-path has to be enabled on both sides of the BGP session, but it could be enabled separately for RX and TX direction. When active, all available routes accepted by the export filter are advertised to the neighbor. Default: off.
allow bgp_local_pref switchA standard BGP implementation do not send the Local Preference attribute to eBGP neighbors and ignore this attribute if received from eBGP neighbors, as per RFC 4271. When this option is enabled on an eBGP session, this attribute will be sent to and accepted from the peer, which is useful for example if you have a setup like in RFC 7938. The option does not affect iBGP sessions. Default: off.
allow local as [number]BGP prevents routing loops by rejecting received routes with the local AS number in the AS path. This option allows to loose or disable the check. Optional
numberargument can be used to specify the maximum number of local ASNs in the AS path that is allowed for received routes. When the option is used without the argument, the check is completely disabled and you should ensure loop-free behavior by some other means. Default: 0 (no local AS number allowed).enable route refresh switchAfter the initial route exchange, BGP protocol uses incremental updates to keep BGP speakers synchronized. Sometimes (e.g., if BGP speaker changes its import filter, or if there is suspicion of inconsistency) it is necessary to do a new complete route exchange. BGP protocol extension Route Refresh (RFC 2918) allows BGP speaker to request re-advertisement of all routes from its neighbor. BGP protocol extension Enhanced Route Refresh (RFC 7313) specifies explicit begin and end for such exchanges, therefore the receiver can remove stale routes that were not advertised during the exchange. This option specifies whether BIRD advertises these capabilities and supports related procedures. Note that even when disabled, BIRD can send route refresh requests. Default: on.
graceful restart switch|awareWhen a BGP speaker restarts or crashes, neighbors will discard all received paths from the speaker, which disrupts packet forwarding even when the forwarding plane of the speaker remains intact. RFC 4724 specifies an optional graceful restart mechanism to alleviate this issue. This option controls the mechanism. It has three states: Disabled, when no support is provided. Aware, when the graceful restart support is announced and the support for restarting neighbors is provided, but no local graceful restart is allowed (i.e. receiving-only role). Enabled, when the full graceful restart support is provided (i.e. both restarting and receiving role). Note that proper support for local graceful restart requires also configuration of other protocols. Default: aware.
graceful restart time numberThe restart time is announced in the BGP graceful restart capability and specifies how long the neighbor would wait for the BGP session to re-establish after a restart before deleting stale routes. Default: 120 seconds.
long lived graceful restart switch|awareThe long-lived graceful restart is an extension of the traditional BGP graceful restart, where stale routes are kept even after the restart time expires for additional long-lived stale time, but they are marked with the LLGR_STALE community, depreferenced, and withdrawn from routers not supporting LLGR. Like traditional BGP graceful restart, it has three states: disabled, aware (receiving-only), and enabled. Note that long-lived graceful restart requires at least aware level of traditional BGP graceful restart. Default: aware, unless graceful restart is disabled.
long lived stale time numberThe long-lived stale time is announced in the BGP long-lived graceful restart capability and specifies how long the neighbor would keep stale routes depreferenced during long-lived graceful restart until either the session is re-stablished and synchronized or the stale time expires and routes are removed. Default: 3600 seconds.
interpret communities switchRFC 1997 demands that BGP speaker should process well-known communities like no-export (65535, 65281) or no-advertise (65535, 65282). For example, received route carrying a no-adverise community should not be advertised to any of its neighbors. If this option is enabled (which is by default), BIRD has such behavior automatically (it is evaluated when a route is exported to the BGP protocol just before the export filter). Otherwise, this integrated processing of well-known communities is disabled. In that case, similar behavior can be implemented in the export filter. Default: on.
enable as4 switchBGP protocol was designed to use 2B AS numbers and was extended later to allow 4B AS number. BIRD supports 4B AS extension, but by disabling this option it can be persuaded not to advertise it and to maintain old-style sessions with its neighbors. This might be useful for circumventing bugs in neighbor's implementation of 4B AS extension. Even when disabled (off), BIRD behaves internally as AS4-aware BGP router. Default: on.
enable extended messages switchThe BGP protocol uses maximum message length of 4096 bytes. This option provides an extension to allow extended messages with length up to 65535 bytes. Default: off.
capabilities switchUse capability advertisement to advertise optional capabilities. This is standard behavior for newer BGP implementations, but there might be some older BGP implementations that reject such connection attempts. When disabled (off), features that request it (4B AS support) are also disabled. Default: on, with automatic fallback to off when received capability-related error.
advertise ipv4 switchAdvertise IPv4 multiprotocol capability. This is not a correct behavior according to the strict interpretation of RFC 4760, but it is widespread and required by some BGP implementations (Cisco and Quagga). This option is relevant to IPv4 mode with enabled capability advertisement only. Default: on.
route limit numberThe maximal number of routes that may be imported from the protocol. If the route limit is exceeded, the connection is closed with an error. Limit is currently implemented as
import limit number action restart. This option is obsolete and it is replaced by import limit option. Default: no limit.disable after error switchWhen an error is encountered (either locally or by the other side), disable the instance automatically and wait for an administrator to fix the problem manually. Default: off.
disable after cease switch|set-of-flagsWhen a Cease notification is received, disable the instance automatically and wait for an administrator to fix the problem manually. When used with switch argument, it means handle every Cease subtype with the exception of
connection collision. Default: off.The set-of-flags allows to narrow down relevant Cease subtypes. The syntax is
{flag [, ...] }, where flags are:cease,prefix limit hit,administrative shutdown,peer deconfigured,administrative reset,connection rejected,configuration change,connection collision,out of resources.hold time numberTime in seconds to wait for a Keepalive message from the other side before considering the connection stale. Default: depends on agreement with the neighboring router, we prefer 240 seconds if the other side is willing to accept it.
startup hold time numberValue of the hold timer used before the routers have a chance to exchange open messages and agree on the real value. Default: 240 seconds.
keepalive time numberDelay in seconds between sending of two consecutive Keepalive messages. Default: One third of the hold time.
connect delay time numberDelay in seconds between protocol startup and the first attempt to connect. Default: 5 seconds.
connect retry time numberTime in seconds to wait before retrying a failed attempt to connect. Default: 120 seconds.
error wait time number,numberMinimum and maximum delay in seconds between a protocol failure (either local or reported by the peer) and automatic restart. Doesn't apply when
disable after erroris configured. If consecutive errors happen, the delay is increased exponentially until it reaches the maximum. Default: 60, 300.error forget time numberMaximum time in seconds between two protocol failures to treat them as a error sequence which makes
error wait timeincrease exponentially. Default: 300 seconds.path metric switchEnable comparison of path lengths when deciding which BGP route is the best one. Default: on.
med metric switchEnable comparison of MED attributes (during best route selection) even between routes received from different ASes. This may be useful if all MED attributes contain some consistent metric, perhaps enforced in import filters of AS boundary routers. If this option is disabled, MED attributes are compared only if routes are received from the same AS (which is the standard behavior). Default: off.
deterministic med switchBGP route selection algorithm is often viewed as a comparison between individual routes (e.g. if a new route appears and is better than the current best one, it is chosen as the new best one). But the proper route selection, as specified by RFC 4271, cannot be fully implemented in that way. The problem is mainly in handling the MED attribute. BIRD, by default, uses an simplification based on individual route comparison, which in some cases may lead to temporally dependent behavior (i.e. the selection is dependent on the order in which routes appeared). This option enables a different (and slower) algorithm implementing proper RFC 4271 route selection, which is deterministic. Alternative way how to get deterministic behavior is to use
med metricoption. This option is incompatible with sorted tables. Default: off.igp metric switchEnable comparison of internal distances to boundary routers during best route selection. Default: on.
prefer older switchStandard route selection algorithm breaks ties by comparing router IDs. This changes the behavior to prefer older routes (when both are external and from different peer). For details, see RFC 5004. Default: off.
default bgp_med numberValue of the Multiple Exit Discriminator to be used during route selection when the MED attribute is missing. Default: 0.
default bgp_local_pref numberA default value for the Local Preference attribute. It is used when a new Local Preference attribute is attached to a route by the BGP protocol itself (for example, if a route is received through eBGP and therefore does not have such attribute). Default: 100 (0 in pre-1.2.0 versions of BIRD).
Attributes
BGP defines several route attributes. Some of them (those marked with
`I' in the table below) are available on internal BGP connections only,
some of them (marked with `O') are optional.
bgppath bgp_pathSequence of AS numbers describing the AS path the packet will travel through when forwarded according to the particular route. In case of internal BGP it doesn't contain the number of the local AS.
int bgp_local_pref [I]Local preference value used for selection among multiple BGP routes (see the selection rules above). It's used as an additional metric which is propagated through the whole local AS.
int bgp_med [O]The Multiple Exit Discriminator of the route is an optional attribute which is used on external (inter-AS) links to convey to an adjacent AS the optimal entry point into the local AS. The received attribute is also propagated over internal BGP links. The attribute value is zeroed when a route is exported to an external BGP instance to ensure that the attribute received from a neighboring AS is not propagated to other neighboring ASes. A new value might be set in the export filter of an external BGP instance. See RFC 4451 for further discussion of BGP MED attribute.
enum bgp_originOrigin of the route: either
ORIGIN_IGPif the route has originated in an interior routing protocol orORIGIN_EGPif it's been imported from theEGPprotocol (nowadays it seems to be obsolete) orORIGIN_INCOMPLETEif the origin is unknown.ip bgp_next_hopNext hop to be used for forwarding of packets to this destination. On internal BGP connections, it's an address of the originating router if it's inside the local AS or a boundary router the packet will leave the AS through if it's an exterior route, so each BGP speaker within the AS has a chance to use the shortest interior path possible to this point.
void bgp_atomic_aggr [O]This is an optional attribute which carries no value, but the sole presence of which indicates that the route has been aggregated from multiple routes by some router on the path from the originator.
clist bgp_community [O]List of community values associated with the route. Each such value is a pair (represented as a
pairdata type inside the filters) of 16-bit integers, the first of them containing the number of the AS which defines the community and the second one being a per-AS identifier. There are lots of uses of the community mechanism, but generally they are used to carry policy information like "don't export to USA peers". As each AS can define its own routing policy, it also has a complete freedom about which community attributes it defines and what will their semantics be.eclist bgp_ext_community [O]List of extended community values associated with the route. Extended communities have similar usage as plain communities, but they have an extended range (to allow 4B ASNs) and a nontrivial structure with a type field. Individual community values are represented using an
ecdata type inside the filters.lclist bgp_large_community [O]List of large community values associated with the route. Large BGP communities is another variant of communities, but contrary to extended communities they behave very much the same way as regular communities, just larger -- they are uniform untyped triplets of 32bit numbers. Individual community values are represented using an
lcdata type inside the filters.quad bgp_originator_id [I, O]This attribute is created by the route reflector when reflecting the route and contains the router ID of the originator of the route in the local AS.
clist bgp_cluster_list [I, O]This attribute contains a list of cluster IDs of route reflectors. Each route reflector prepends its cluster ID when reflecting the route.
Example
protocol bgp {
local as 65000; # Use a private AS number
neighbor 198.51.100.130 as 64496; # Our neighbor ...
multihop; # ... which is connected indirectly
export filter { # We use non-trivial export rules
if source = RTS_STATIC then { # Export only static routes
# Assign our community
bgp_community.add((65000,64501));
# Artificially increase path length
# by advertising local AS number twice
if bgp_path ~ [= 65000 =] then
bgp_path.prepend(65000);
accept;
}
reject;
};
import all;
source address 198.51.100.14; # Use a non-standard source address
}
6.4 Device
The Device protocol is not a real routing protocol. It doesn't generate any routes and it only serves as a module for getting information about network interfaces from the kernel.
Except for very unusual circumstances, you probably should include this protocol in the configuration since almost all other protocols require network interfaces to be defined for them to work with.
Configuration
scan time numberTime in seconds between two scans of the network interface list. On systems where we are notified about interface status changes asynchronously (such as newer versions of Linux), we need to scan the list only in order to avoid confusion by lost notification messages, so the default time is set to a large value.
primary [ "mask" ] prefixIf a network interface has more than one network address, BIRD has to choose one of them as a primary one. By default, BIRD chooses the lexicographically smallest address as the primary one.
This option allows to specify which network address should be chosen as a primary one. Network addresses that match prefix are preferred to non-matching addresses. If more
primaryoptions are used, the first one has the highest preference. If "mask" is specified, then suchprimaryoption is relevant only to matching network interfaces.In all cases, an address marked by operating system as secondary cannot be chosen as the primary one.
As the Device protocol doesn't generate any routes, it cannot have any attributes. Example configuration looks like this:
protocol device {
scan time 10; # Scan the interfaces often
primary "eth0" 192.168.1.1;
primary 192.168.0.0/16;
}
6.5 Direct
The Direct protocol is a simple generator of device routes for all the directly connected networks according to the list of interfaces provided by the kernel via the Device protocol.
The question is whether it is a good idea to have such device routes in BIRD routing table. OS kernel usually handles device routes for directly connected networks by itself so we don't need (and don't want) to export these routes to the kernel protocol. OSPF protocol creates device routes for its interfaces itself and BGP protocol is usually used for exporting aggregate routes. Although there are some use cases that use the direct protocol (like abusing eBGP as an IGP routing protocol), in most cases it is not needed to have these device routes in BIRD routing table and to use the direct protocol.
There is one notable case when you definitely want to use the direct protocol -- running BIRD on BSD systems. Having high priority device routes for directly connected networks from the direct protocol protects kernel device routes from being overwritten or removed by IGP routes during some transient network conditions, because a lower priority IGP route for the same network is not exported to the kernel routing table. This is an issue on BSD systems only, as on Linux systems BIRD cannot change non-BIRD route in the kernel routing table.
There are just few configuration options for the Direct protocol:
interface pattern [, ...]By default, the Direct protocol will generate device routes for all the interfaces available. If you want to restrict it to some subset of interfaces or addresses (e.g. if you're using multiple routing tables for policy routing and some of the policy domains don't contain all interfaces), just use this clause. See interface common option for detailed description. The Direct protocol uses extended interface clauses.
check link switchIf enabled, a hardware link state (reported by OS) is taken into consideration. Routes for directly connected networks are generated only if link up is reported and they are withdrawn when link disappears (e.g., an ethernet cable is unplugged). Default value is no.
Direct device routes don't contain any specific attributes.
Example config might look like this:
protocol direct {
interface "-arc*", "*"; # Exclude the ARCnets
}
6.6 Kernel
The Kernel protocol is not a real routing protocol. Instead of communicating
with other routers in the network, it performs synchronization of BIRD's routing
tables with the OS kernel. Basically, it sends all routing table updates to the
kernel and from time to time it scans the kernel tables to see whether some
routes have disappeared (for example due to unnoticed up/down transition of an
interface) or whether an `alien' route has been added by someone else (depending
on the learn switch, such routes are either ignored or accepted to our
table).
Unfortunately, there is one thing that makes the routing table synchronization
a bit more complicated. In the kernel routing table there are also device routes
for directly connected networks. These routes are usually managed by OS itself
(as a part of IP address configuration) and we don't want to touch that. They
are completely ignored during the scan of the kernel tables and also the export
of device routes from BIRD tables to kernel routing tables is restricted to
prevent accidental interference. This restriction can be disabled using
device routes switch.
If your OS supports only a single routing table, you can configure only one instance of the Kernel protocol. If it supports multiple tables (in order to allow policy routing; such an OS is for example Linux), you can run as many instances as you want, but each of them must be connected to a different BIRD routing table and to a different kernel table.
Because the kernel protocol is partially integrated with the connected routing table, there are two limitations - it is not possible to connect more kernel protocols to the same routing table and changing route destination (gateway) in an export filter of a kernel protocol does not work. Both limitations can be overcome using another routing table and the pipe protocol.
Configuration
persist switchTell BIRD to leave all its routes in the routing tables when it exits (instead of cleaning them up).
scan time numberTime in seconds between two consecutive scans of the kernel routing table.
learn switchEnable learning of routes added to the kernel routing tables by other routing daemons or by the system administrator. This is possible only on systems which support identification of route authorship.
device routes switchEnable export of device routes to the kernel routing table. By default, such routes are rejected (with the exception of explicitly configured device routes from the static protocol) regardless of the export filter to protect device routes in kernel routing table (managed by OS itself) from accidental overwriting or erasing.
kernel table numberSelect which kernel table should this particular instance of the Kernel protocol work with. Available only on systems supporting multiple routing tables.
metric number(Linux) Use specified value as a kernel metric (priority) for all routes sent to the kernel. When multiple routes for the same network are in the kernel routing table, the Linux kernel chooses one with lower metric. Also, routes with different metrics do not clash with each other, therefore using dedicated metric value is a reliable way to avoid overwriting routes from other sources (e.g. kernel device routes). Metric 0 has a special meaning of undefined metric, in which either OS default is used, or per-route metric can be set using
krt_metricattribute. Default: 0 (undefined).graceful restart switchParticipate in graceful restart recovery. If this option is enabled and a graceful restart recovery is active, the Kernel protocol will defer synchronization of routing tables until the end of the recovery. Note that import of kernel routes to BIRD is not affected.
merge paths switch [limit number]Usually, only best routes are exported to the kernel protocol. With path merging enabled, both best routes and equivalent non-best routes are merged during export to generate one ECMP (equal-cost multipath) route for each network. This is useful e.g. for BGP multipath. Note that best routes are still pivotal for route export (responsible for most properties of resulting ECMP routes), while exported non-best routes are responsible just for additional multipath next hops. This option also allows to specify a limit on maximal number of nexthops in one route. By default, multipath merging is disabled. If enabled, default value of the limit is 16.
Attributes
The Kernel protocol defines several attributes. These attributes are translated to appropriate system (and OS-specific) route attributes. We support these attributes:
int krt_sourceThe original source of the imported kernel route. The value is system-dependent. On Linux, it is a value of the protocol field of the route. See /etc/iproute2/rt_protos for common values. On BSD, it is based on STATIC and PROTOx flags. The attribute is read-only.
int krt_metric(Linux) The kernel metric of the route. When multiple same routes are in a kernel routing table, the Linux kernel chooses one with lower metric. Note that preferred way to set kernel metric is to use protocol option
metric, unless per-route metric values are needed.ip krt_prefsrc(Linux) The preferred source address. Used in source address selection for outgoing packets. Has to be one of the IP addresses of the router.
int krt_realm(Linux) The realm of the route. Can be used for traffic classification.
int krt_scope(Linux IPv4) The scope of the route. Valid values are 0-254, although Linux kernel may reject some values depending on route type and nexthop. It is supposed to represent `indirectness' of the route, where nexthops of routes are resolved through routes with a higher scope, but in current kernels anything below link (253) is treated as global (0). When not present, global scope is implied for all routes except device routes, where link scope is used by default.
In Linux, there is also a plenty of obscure route attributes mostly focused
on tuning TCP performance of local connections. BIRD supports most of these
attributes, see Linux or iproute2 documentation for their meaning. Attributes
krt_lock_* and krt_feature_* have type bool, others have type int.
Supported attributes are:
krt_mtu, krt_lock_mtu, krt_window, krt_lock_window,
krt_rtt, krt_lock_rtt, krt_rttvar, krt_lock_rttvar,
krt_sstresh, krt_lock_sstresh, krt_cwnd, krt_lock_cwnd,
krt_advmss, krt_lock_advmss, krt_reordering, krt_lock_reordering,
krt_hoplimit, krt_lock_hoplimit, krt_rto_min, krt_lock_rto_min,
krt_initcwnd, krt_initrwnd, krt_quickack,
krt_feature_ecn, krt_feature_allfrag
Example
A simple configuration can look this way:
protocol kernel {
export all;
}
Or for a system with two routing tables:
protocol kernel { # Primary routing table
learn; # Learn alien routes from the kernel
persist; # Don't remove routes on bird shutdown
scan time 10; # Scan kernel routing table every 10 seconds
import all;
export all;
}
protocol kernel { # Secondary routing table
table auxtable;
kernel table 100;
export all;
}
6.7 MRT
Introduction
The MRT protocol is a component responsible for handling the Multi-Threaded Routing Toolkit (MRT) routing information export format, which is mainly used for collecting and analyzing of routing information from BGP routers. The MRT protocol can be configured to do periodic dumps of routing tables, created MRT files can be analyzed later by other tools. Independent MRT table dumps can also be requested from BIRD client. There is also a feature to save incoming BGP messages in MRT files, but it is controlled by mrtdump options independently of MRT protocol, although that might change in the future.
BIRD implements the main MRT format specification as defined in RFC 6396 and the ADD_PATH extension (RFC 8050).
Configuration
MRT configuration consists of several statements describing routing table
dumps. Multiple independent periodic dumps can be done as multiple MRT protocol
instances. There are two mandatory statements: filename and period.
The behavior can be modified by following configuration parameters:
table name | "pattern"Specify a routing table (or a set of routing tables described by a wildcard pattern) that are to be dumped by the MRT protocol instance. Default: the master table.
filter { filter commands }The MRT protocol allows to specify a filter that is applied to routes as they are dumped. Rejected routes are ignored and not saved to the MRT dump file. Default: no filter.
where filter expressionAn alternative way to specify a filter for the MRT protocol.
filename "filename"Specify a filename for MRT dump files. The filename may contain time format sequences with strftime(3) notation (see man strftime for details), there is also a sequence "%N" that is expanded to the name of dumped table. Therefore, each periodic dump of each table can be saved to a different file. Mandatory, see example below.
period numberSpecify the time interval (in seconds) between periodic dumps. Mandatory.
always add path switchThe MRT format uses special records (specified in RFC 8050) for routes received using BGP ADD_PATH extension to keep Path ID, while other routes use regular records. This has advantage of better compatibility with tools that do not know special records, but it loses information about which route is the best route. When this option is enabled, both ADD_PATH and non-ADD_PATH routes are stored in ADD_PATH records and order of routes for network is preserved. Default: disabled.
Example
protocol mrt {
table "tab*";
where source = RTS_BGP;
filename "/var/log/bird/%N_%F_%T.mrt";
period 300;
}
6.8 OSPF
Introduction
Open Shortest Path First (OSPF) is a quite complex interior gateway protocol. The current IPv4 version (OSPFv2) is defined in RFC 2328 and the current IPv6 version (OSPFv3) is defined in RFC 5340 It's a link state (a.k.a. shortest path first) protocol -- each router maintains a database describing the autonomous system's topology. Each participating router has an identical copy of the database and all routers run the same algorithm calculating a shortest path tree with themselves as a root. OSPF chooses the least cost path as the best path.
In OSPF, the autonomous system can be split to several areas in order to reduce the amount of resources consumed for exchanging the routing information and to protect the other areas from incorrect routing data. Topology of the area is hidden to the rest of the autonomous system.
Another very important feature of OSPF is that it can keep routing information from other protocols (like Static or BGP) in its link state database as external routes. Each external route can be tagged by the advertising router, making it possible to pass additional information between routers on the boundary of the autonomous system.
OSPF quickly detects topological changes in the autonomous system (such as router interface failures) and calculates new loop-free routes after a short period of convergence. Only a minimal amount of routing traffic is involved.
Each router participating in OSPF routing periodically sends Hello messages to all its interfaces. This allows neighbors to be discovered dynamically. Then the neighbors exchange theirs parts of the link state database and keep it identical by flooding updates. The flooding process is reliable and ensures that each router detects all changes.
Configuration
In the main part of configuration, there can be multiple definitions of OSPF areas, each with a different id. These definitions includes many other switches and multiple definitions of interfaces. Definition of interface may contain many switches and constant definitions and list of neighbors on nonbroadcast networks.
protocol ospf <name> {
rfc1583compat <switch>;
instance id <num>;
stub router <switch>;
tick <num>;
ecmp <switch> [limit <num>];
merge external <switch>;
area <id> {
stub;
nssa;
summary <switch>;
default nssa <switch>;
default cost <num>;
default cost2 <num>;
translator <switch>;
translator stability <num>;
networks {
<prefix>;
<prefix> hidden;
}
external {
<prefix>;
<prefix> hidden;
<prefix> tag <num>;
}
stubnet <prefix>;
stubnet <prefix> {
hidden <switch>;
summary <switch>;
cost <num>;
}
interface <interface pattern> [instance <num>] {
cost <num>;
stub <switch>;
hello <num>;
poll <num>;
retransmit <num>;
priority <num>;
wait <num>;
dead count <num>;
dead <num>;
secondary <switch>;
rx buffer [normal|large|<num>];
tx length <num>;
type [broadcast|bcast|pointopoint|ptp|
nonbroadcast|nbma|pointomultipoint|ptmp];
link lsa suppression <switch>;
strict nonbroadcast <switch>;
real broadcast <switch>;
ptp netmask <switch>;
check link <switch>;
bfd <switch>;
ecmp weight <num>;
ttl security [<switch>; | tx only]
tx class|dscp <num>;
tx priority <num>;
authentication none|simple|cryptographic;
password "<text>";
password "<text>" {
id <num>;
generate from "<date>";
generate to "<date>";
accept from "<date>";
accept to "<date>";
from "<date>";
to "<date>";
algorithm ( keyed md5 | keyed sha1 | hmac sha1 | hmac sha256 | hmac sha384 | hmac sha512 );
};
neighbors {
<ip>;
<ip> eligible;
};
};
virtual link <id> [instance <num>] {
hello <num>;
retransmit <num>;
wait <num>;
dead count <num>;
dead <num>;
authentication none|simple|cryptographic;
password "<text>";
password "<text>" {
id <num>;
generate from "<date>";
generate to "<date>";
accept from "<date>";
accept to "<date>";
from "<date>";
to "<date>";
algorithm ( keyed md5 | keyed sha1 | hmac sha1 | hmac sha256 | hmac sha384 | hmac sha512 );
};
};
};
}
rfc1583compat switchThis option controls compatibility of routing table calculation with RFC 1583. Default value is no.
instance id numWhen multiple OSPF protocol instances are active on the same links, they should use different instance IDs to distinguish their packets. Although it could be done on per-interface basis, it is often preferred to set one instance ID to whole OSPF domain/topology (e.g., when multiple instances are used to represent separate logical topologies on the same physical network). This option specifies the default instance ID for all interfaces of the OSPF instance. Note that this option, if used, must precede interface definitions. Default value is 0.
stub router switchThis option configures the router to be a stub router, i.e., a router that participates in the OSPF topology but does not allow transit traffic. In OSPFv2, this is implemented by advertising maximum metric for outgoing links. In OSPFv3, the stub router behavior is announced by clearing the R-bit in the router LSA. See RFC 6987 for details. Default value is no.
tick numThe routing table calculation and clean-up of areas' databases is not performed when a single link state change arrives. To lower the CPU utilization, it's processed later at periodical intervals of num seconds. The default value is 1.
ecmp switch [limit number]This option specifies whether OSPF is allowed to generate ECMP (equal-cost multipath) routes. Such routes are used when there are several directions to the destination, each with the same (computed) cost. This option also allows to specify a limit on maximum number of nexthops in one route. By default, ECMP is disabled. If enabled, default value of the limit is 16.
merge external switchThis option specifies whether OSPF should merge external routes from different routers/LSAs for the same destination. When enabled together with
ecmp, equal-cost external routes will be combined to multipath routes in the same way as regular routes. When disabled, external routes from different LSAs are treated as separate even if they represents the same destination. Default value is no.area idThis defines an OSPF area with given area ID (an integer or an IPv4 address, similarly to a router ID). The most important area is the backbone (ID 0) to which every other area must be connected.
stubThis option configures the area to be a stub area. External routes are not flooded into stub areas. Also summary LSAs can be limited in stub areas (see option
summary). By default, the area is not a stub area.nssaThis option configures the area to be a NSSA (Not-So-Stubby Area). NSSA is a variant of a stub area which allows a limited way of external route propagation. Global external routes are not propagated into a NSSA, but an external route can be imported into NSSA as a (area-wide) NSSA-LSA (and possibly translated and/or aggregated on area boundary). By default, the area is not NSSA.
summary switchThis option controls propagation of summary LSAs into stub or NSSA areas. If enabled, summary LSAs are propagated as usual, otherwise just the default summary route (0.0.0.0/0) is propagated (this is sometimes called totally stubby area). If a stub area has more area boundary routers, propagating summary LSAs could lead to more efficient routing at the cost of larger link state database. Default value is no.
default nssa switchWhen
summaryoption is enabled, default summary route is no longer propagated to the NSSA. In that case, this option allows to originate default route as NSSA-LSA to the NSSA. Default value is no.default cost numThis option controls the cost of a default route propagated to stub and NSSA areas. Default value is 1000.
default cost2 numWhen a default route is originated as NSSA-LSA, its cost can use either type 1 or type 2 metric. This option allows to specify the cost of a default route in type 2 metric. By default, type 1 metric (option
default cost) is used.translator switchThis option controls translation of NSSA-LSAs into external LSAs. By default, one translator per NSSA is automatically elected from area boundary routers. If enabled, this area boundary router would unconditionally translate all NSSA-LSAs regardless of translator election. Default value is no.
translator stability numThis option controls the translator stability interval (in seconds). When the new translator is elected, the old one keeps translating until the interval is over. Default value is 40.
networks { set }Definition of area IP ranges. This is used in summary LSA origination. Hidden networks are not propagated into other areas.
external { set }Definition of external area IP ranges for NSSAs. This is used for NSSA-LSA translation. Hidden networks are not translated into external LSAs. Networks can have configured route tag.
stubnet prefix { options }Stub networks are networks that are not transit networks between OSPF routers. They are also propagated through an OSPF area as a part of a link state database. By default, BIRD generates a stub network record for each primary network address on each OSPF interface that does not have any OSPF neighbors, and also for each non-primary network address on each OSPF interface. This option allows to alter a set of stub networks propagated by this router.
Each instance of this option adds a stub network with given network prefix to the set of propagated stub network, unless option
hiddenis used. It also suppresses default stub networks for given network prefix. When optionsummaryis used, also default stub networks that are subnetworks of given stub network are suppressed. This might be used, for example, to aggregate generated stub networks.interface pattern [instance num]Defines that the specified interfaces belong to the area being defined. See interface common option for detailed description. In OSPFv2, extended interface clauses are used, because each network prefix is handled as a separate virtual interface.
You can specify alternative instance ID for the interface definition, therefore it is possible to have several instances of that interface with different options or even in different areas. For OSPFv2, instance ID support is an extension (RFC 6549) and is supposed to be set per-protocol. For OSPFv3, it is an integral feature.
virtual link id [instance num]Virtual link to router with the router id. Virtual link acts as a point-to-point interface belonging to backbone. The actual area is used as a transport area. This item cannot be in the backbone. Like with
interfaceoption, you could also use several virtual links to one destination with different instance IDs.cost numSpecifies output cost (metric) of an interface. Default value is 10.
stub switchIf set to interface it does not listen to any packet and does not send any hello. Default value is no.
hello numSpecifies interval in seconds between sending of Hello messages. Beware, all routers on the same network need to have the same hello interval. Default value is 10.
poll numSpecifies interval in seconds between sending of Hello messages for some neighbors on NBMA network. Default value is 20.
retransmit numSpecifies interval in seconds between retransmissions of unacknowledged updates. Default value is 5.
transmit delay numSpecifies estimated transmission delay of link state updates send over the interface. The value is added to LSA age of LSAs propagated through it. Default value is 1.
priority numOn every multiple access network (e.g., the Ethernet) Designated Router and Backup Designated router are elected. These routers have some special functions in the flooding process. Higher priority increases preferences in this election. Routers with priority 0 are not eligible. Default value is 1.
wait numAfter start, router waits for the specified number of seconds between starting election and building adjacency. Default value is 4*hello.
dead count numWhen the router does not receive any messages from a neighbor in dead count*hello seconds, it will consider the neighbor down.
dead numWhen the router does not receive any messages from a neighbor in dead seconds, it will consider the neighbor down. If both directives
dead countanddeadare used,deadhas precedence.secondary switchOn BSD systems, older versions of BIRD supported OSPFv2 only for the primary IP address of an interface, other IP ranges on the interface were handled as stub networks. Since v1.4.1, regular operation on secondary IP addresses is supported, but disabled by default for compatibility. This option allows to enable it. The option is a transitional measure, will be removed in the next major release as the behavior will be changed. On Linux systems, the option is irrelevant, as operation on non-primary addresses is already the regular behavior.
rx buffer numThis option allows to specify the size of buffers used for packet processing. The buffer size should be bigger than maximal size of any packets. By default, buffers are dynamically resized as needed, but a fixed value could be specified. Value
largemeans maximal allowed packet size - 65535.tx length numTransmitted OSPF messages that contain large amount of information are segmented to separate OSPF packets to avoid IP fragmentation. This option specifies the soft ceiling for the length of generated OSPF packets. Default value is the MTU of the network interface. Note that larger OSPF packets may still be generated if underlying OSPF messages cannot be splitted (e.g. when one large LSA is propagated).
type broadcast|bcastBIRD detects a type of a connected network automatically, but sometimes it's convenient to force use of a different type manually. On broadcast networks (like ethernet), flooding and Hello messages are sent using multicasts (a single packet for all the neighbors). A designated router is elected and it is responsible for synchronizing the link-state databases and originating network LSAs. This network type cannot be used on physically NBMA networks and on unnumbered networks (networks without proper IP prefix).
type pointopoint|ptpPoint-to-point networks connect just 2 routers together. No election is performed and no network LSA is originated, which makes it simpler and faster to establish. This network type is useful not only for physically PtP ifaces (like PPP or tunnels), but also for broadcast networks used as PtP links. This network type cannot be used on physically NBMA networks.
type nonbroadcast|nbmaOn NBMA networks, the packets are sent to each neighbor separately because of lack of multicast capabilities. Like on broadcast networks, a designated router is elected, which plays a central role in propagation of LSAs. This network type cannot be used on unnumbered networks.
type pointomultipoint|ptmpThis is another network type designed to handle NBMA networks. In this case the NBMA network is treated as a collection of PtP links. This is useful if not every pair of routers on the NBMA network has direct communication, or if the NBMA network is used as an (possibly unnumbered) PtP link.
link lsa suppression switchIn OSPFv3, link LSAs are generated for each link, announcing link-local IPv6 address of the router to its local neighbors. These are useless on PtP or PtMP networks and this option allows to suppress the link LSA origination for such interfaces. The option is ignored on other than PtP or PtMP interfaces. Default value is no.
strict nonbroadcast switchIf set, don't send hello to any undefined neighbor. This switch is ignored on other than NBMA or PtMP interfaces. Default value is no.
real broadcast switchIn
type broadcastortype ptpnetwork configuration, OSPF packets are sent as IP multicast packets. This option changes the behavior to using old-fashioned IP broadcast packets. This may be useful as a workaround if IP multicast for some reason does not work or does not work reliably. This is a non-standard option and probably is not interoperable with other OSPF implementations. Default value is no.ptp netmask switchIn
type ptpnetwork configurations, OSPFv2 implementations should ignore received netmask field in hello packets and should send hello packets with zero netmask field on unnumbered PtP links. But some OSPFv2 implementations perform netmask checking even for PtP links. This option specifies whether real netmask will be used in hello packets ontype ptpinterfaces. You should ignore this option unless you meet some compatibility problems related to this issue. Default value is no for unnumbered PtP links, yes otherwise.check link switchIf set, a hardware link state (reported by OS) is taken into consideration. When a link disappears (e.g. an ethernet cable is unplugged), neighbors are immediately considered unreachable and only the address of the iface (instead of whole network prefix) is propagated. It is possible that some hardware drivers or platforms do not implement this feature. Default value is no.
bfd switchOSPF could use BFD protocol as an advisory mechanism for neighbor liveness and failure detection. If enabled, BIRD setups a BFD session for each OSPF neighbor and tracks its liveness by it. This has an advantage of an order of magnitude lower detection times in case of failure. Note that BFD protocol also has to be configured, see BFD section for details. Default value is no.
ttl security [switch | tx only]TTL security is a feature that protects routing protocols from remote spoofed packets by using TTL 255 instead of TTL 1 for protocol packets destined to neighbors. Because TTL is decremented when packets are forwarded, it is non-trivial to spoof packets with TTL 255 from remote locations. Note that this option would interfere with OSPF virtual links.
If this option is enabled, the router will send OSPF packets with TTL 255 and drop received packets with TTL less than 255. If this option si set to
tx only, TTL 255 is used for sent packets, but is not checked for received packets. Default value is no.tx class|dscp|priority numThese options specify the ToS/DiffServ/Traffic class/Priority of the outgoing OSPF packets. See tx class common option for detailed description.
ecmp weight numWhen ECMP (multipath) routes are allowed, this value specifies a relative weight used for nexthops going through the iface. Allowed values are 1-256. Default value is 1.
authentication noneNo passwords are sent in OSPF packets. This is the default value.
authentication simpleEvery packet carries 8 bytes of password. Received packets lacking this password are ignored. This authentication mechanism is very weak. This option is not available in OSPFv3.
authentication cryptographicAn authentication code is appended to every packet. The specific cryptographic algorithm is selected by option
algorithmfor each key. The default cryptographic algorithm for OSPFv2 keys is Keyed-MD5 and for OSPFv3 keys is HMAC-SHA-256. Passwords are not sent open via network, so this mechanism is quite secure. Packets can still be read by an attacker.password "text"Specifies a password used for authentication. See password common option for detailed description.
neighbors { set }A set of neighbors to which Hello messages on NBMA or PtMP networks are to be sent. For NBMA networks, some of them could be marked as eligible. In OSPFv3, link-local addresses should be used, using global ones is possible, but it is nonstandard and might be problematic. And definitely, link-local and global addresses should not be mixed.
Attributes
OSPF defines four route attributes. Each internal route has a metric.
Metric is ranging from 1 to infinity (65535). External routes use
metric type 1 or metric type 2. A metric of type 1 is comparable
with internal metric, a metric of type 2 is always longer than any
metric of type 1 or any internal metric. Internal metric or
metric of type 1 is stored in attribute ospf_metric1, metric type
2 is stored in attribute ospf_metric2. If you specify both metrics only
metric1 is used.
Each external route can also carry attribute ospf_tag which is a 32-bit
integer which is used when exporting routes to other protocols; otherwise, it
doesn't affect routing inside the OSPF domain at all. The fourth attribute
ospf_router_id is a router ID of the router advertising that route /
network. This attribute is read-only. Default is ospf_metric2 = 10000 and
ospf_tag = 0.
Example
protocol ospf MyOSPF {
rfc1583compat yes;
tick 2;
export filter {
if source = RTS_BGP then {
ospf_metric1 = 100;
accept;
}
reject;
};
area 0.0.0.0 {
interface "eth*" {
cost 11;
hello 15;
priority 100;
retransmit 7;
authentication simple;
password "aaa";
};
interface "ppp*" {
cost 100;
authentication cryptographic;
password "abc" {
id 1;
generate to "22-04-2003 11:00:06";
accept from "17-01-2001 12:01:05";
algorithm hmac sha384;
};
password "def" {
id 2;
generate to "22-07-2005 17:03:21";
accept from "22-02-2001 11:34:06";
algorithm hmac sha512;
};
};
interface "arc0" {
cost 10;
stub yes;
};
interface "arc1";
};
area 120 {
stub yes;
networks {
172.16.1.0/24;
172.16.2.0/24 hidden;
}
interface "-arc0" , "arc*" {
type nonbroadcast;
authentication none;
strict nonbroadcast yes;
wait 120;
poll 40;
dead count 8;
neighbors {
192.168.120.1 eligible;
192.168.120.2;
192.168.120.10;
};
};
};
}
6.9 Pipe
Introduction
The Pipe protocol serves as a link between two routing tables, allowing
routes to be passed from a table declared as primary (i.e., the one the pipe is
connected to using the table configuration keyword) to the secondary one
(declared using peer table) and vice versa, depending on what's allowed by
the filters. Export filters control export of routes from the primary table to
the secondary one, import filters control the opposite direction.
The Pipe protocol may work in the transparent mode mode or in the opaque mode. In the transparent mode, the Pipe protocol retransmits all routes from one table to the other table, retaining their original source and attributes. If import and export filters are set to accept, then both tables would have the same content. The transparent mode is the default mode.
In the opaque mode, the Pipe protocol retransmits optimal route from one
table to the other table in a similar way like other protocols send and receive
routes. Retransmitted route will have the source set to the Pipe protocol, which
may limit access to protocol specific route attributes. This mode is mainly for
compatibility, it is not suggested for new configs. The mode can be changed by
mode option.
The primary use of multiple routing tables and the Pipe protocol is for
policy routing, where handling of a single packet doesn't depend only on its
destination address, but also on its source address, source interface, protocol
type and other similar parameters. In many systems (Linux being a good example),
the kernel allows to enforce routing policies by defining routing rules which
choose one of several routing tables to be used for a packet according to its
parameters. Setting of these rules is outside the scope of BIRD's work (on
Linux, you can use the ip command), but you can create several routing
tables in BIRD, connect them to the kernel ones, use filters to control which
routes appear in which tables and also you can employ the Pipe protocol for
exporting a selected subset of one table to another one.
Configuration
peer table tableDefines secondary routing table to connect to. The primary one is selected by the
tablekeyword.mode opaque|transparentSpecifies the mode for the pipe to work in. Default is transparent.
Attributes
The Pipe protocol doesn't define any route attributes.
Example
Let's consider a router which serves as a boundary router of two different autonomous systems, each of them connected to a subset of interfaces of the router, having its own exterior connectivity and wishing to use the other AS as a backup connectivity in case of outage of its own exterior line.
Probably the simplest solution to this situation is to use two routing tables
(we'll call them as1 and as2) and set up kernel routing rules, so that
packets having arrived from interfaces belonging to the first AS will be routed
according to as1 and similarly for the second AS. Thus we have split our
router to two logical routers, each one acting on its own routing table, having
its own routing protocols on its own interfaces. In order to use the other AS's
routes for backup purposes, we can pass the routes between the tables through a
Pipe protocol while decreasing their preferences and correcting their BGP paths
to reflect the AS boundary crossing.
table as1; # Define the tables
table as2;
protocol kernel kern1 { # Synchronize them with the kernel
table as1;
kernel table 1;
}
protocol kernel kern2 {
table as2;
kernel table 2;
}
protocol bgp bgp1 { # The outside connections
table as1;
local as 1;
neighbor 192.168.0.1 as 1001;
export all;
import all;
}
protocol bgp bgp2 {
table as2;
local as 2;
neighbor 10.0.0.1 as 1002;
export all;
import all;
}
protocol pipe { # The Pipe
table as1;
peer table as2;
export filter {
if net ~ [ 1.0.0.0/8+] then { # Only AS1 networks
if preference>10 then preference = preference-10;
if source=RTS_BGP then bgp_path.prepend(1);
accept;
}
reject;
};
import filter {
if net ~ [ 2.0.0.0/8+] then { # Only AS2 networks
if preference>10 then preference = preference-10;
if source=RTS_BGP then bgp_path.prepend(2);
accept;
}
reject;
};
}
6.10 RAdv
Introduction
The RAdv protocol is an implementation of Router Advertisements, which are used in the IPv6 stateless autoconfiguration. IPv6 routers send (in irregular time intervals or as an answer to a request) advertisement packets to connected networks. These packets contain basic information about a local network (e.g. a list of network prefixes), which allows network hosts to autoconfigure network addresses and choose a default route. BIRD implements router behavior as defined in RFC 4861, router preferences and specific routes (RFC 4191), and DNS extensions (RFC 6106).
Configuration
There are several classes of definitions in RAdv configuration -- interface definitions, prefix definitions and DNS definitions:
interface pattern [, ...] { options }Interface definitions specify a set of interfaces on which the protocol is activated and contain interface specific options. See interface common options for detailed description.
prefix prefix { options }Prefix definitions allow to modify a list of advertised prefixes. By default, the advertised prefixes are the same as the network prefixes assigned to the interface. For each network prefix, the matching prefix definition is found and its options are used. If no matching prefix definition is found, the prefix is used with default options.
Prefix definitions can be either global or interface-specific. The second ones are part of interface options. The prefix definition matching is done in the first-match style, when interface-specific definitions are processed before global definitions. As expected, the prefix definition is matching if the network prefix is a subnet of the prefix in prefix definition.
rdnss { options }RDNSS definitions allow to specify a list of advertised recursive DNS servers together with their options. As options are seldom necessary, there is also a short variant
rdnss addressthat just specifies one DNS server. Multiple definitions are cumulative. RDNSS definitions may also be interface-specific when used inside interface options. By default, interface uses both global and interface-specific options, but that can be changed byrdnss localoption.dnssl { options }DNSSL definitions allow to specify a list of advertised DNS search domains together with their options. Like
rdnssabove, multiple definitions are cumulative, they can be used also as interface-specific options and there is a short variantdnssl domainthat just specifies one DNS search domain.trigger prefixRAdv protocol could be configured to change its behavior based on availability of routes. When this option is used, the protocol waits in suppressed state until a trigger route (for the specified network) is exported to the protocol, the protocol also returns to suppressed state if the trigger route disappears. Note that route export depends on specified export filter, as usual. This option could be used, e.g., for handling failover in multihoming scenarios.
During suppressed state, router advertisements are generated, but with some fields zeroed. Exact behavior depends on which fields are zeroed, this can be configured by
sensitiveoption for appropriate fields. By default, justdefault lifetime(also calledrouter lifetime) is zeroed, which means hosts cannot use the router as a default router.preferred lifetimeandvalid lifetimecould also be configured assensitivefor a prefix, which would cause autoconfigured IPs to be deprecated or even removed.propagate routes switchThis option controls propagation of more specific routes, as defined in RFC 4191. If enabled, all routes exported to the RAdv protocol, with the exception of the trigger prefix, are added to advertisments as additional options. The lifetime and preference of advertised routes can be set individually by
ra_lifetimeandra_preferenceroute attributes, or per interface byroute lifetimeandroute preferenceoptions. Default: disabled.Note that the RFC discourages from sending more than 17 routes and recommends the routes to be configured manually.
Interface specific options:
max ra interval exprUnsolicited router advertisements are sent in irregular time intervals. This option specifies the maximum length of these intervals, in seconds. Valid values are 4-1800. Default: 600
min ra interval exprThis option specifies the minimum length of that intervals, in seconds. Must be at least 3 and at most 3/4 *
max ra interval. Default: about 1/3 *max ra interval.min delay exprThe minimum delay between two consecutive router advertisements, in seconds. Default: 3
managed switchThis option specifies whether hosts should use DHCPv6 for IP address configuration. Default: no
other config switchThis option specifies whether hosts should use DHCPv6 to receive other configuration information. Default: no
link mtu exprThis option specifies which value of MTU should be used by hosts. 0 means unspecified. Default: 0
reachable time exprThis option specifies the time (in milliseconds) how long hosts should assume a neighbor is reachable (from the last confirmation). Maximum is 3600000, 0 means unspecified. Default 0.
retrans timer exprThis option specifies the time (in milliseconds) how long hosts should wait before retransmitting Neighbor Solicitation messages. 0 means unspecified. Default 0.
current hop limit exprThis option specifies which value of Hop Limit should be used by hosts. Valid values are 0-255, 0 means unspecified. Default: 64
default lifetime expr [sensitive switch]This option specifies the time (in seconds) how long (since the receipt of RA) hosts may use the router as a default router. 0 means do not use as a default router. For
sensitiveoption, see trigger. Default: 3 *max ra interval,sensitiveyes.default preference low|medium|highThis option specifies the Default Router Preference value to advertise to hosts. Default: medium.
route lifetime expr [sensitive switch]This option specifies the default value of advertised lifetime for specific routes; i.e., the time (in seconds) for how long (since the receipt of RA) hosts should consider these routes valid. A special value 0xffffffff represents infinity. The lifetime can be overriden on a per route basis by the ra_lifetime route attribute. Default: 3 *
max ra interval,sensitiveno.For the
sensitiveoption, see trigger. Ifsensitiveis enabled, even the routes with thera_lifetimeattribute become sensitive to the trigger.route preference low|medium|highThis option specifies the default value of advertised route preference for specific routes. The value can be overriden on a per route basis by the ra_preference route attribute. Default: medium.
prefix linger time exprWhen a prefix or a route disappears, it is advertised for some time with zero lifetime, to inform clients it is no longer valid. This option specifies the time (in seconds) for how long prefixes are advertised that way. Default: 3 *
max ra interval.route linger time exprWhen a prefix or a route disappears, it is advertised for some time with zero lifetime, to inform clients it is no longer valid. This option specifies the time (in seconds) for how long routes are advertised that way. Default: 3 *
max ra interval.rdnss local switchUse only local (interface-specific) RDNSS definitions for this interface. Otherwise, both global and local definitions are used. Could also be used to disable RDNSS for given interface if no local definitons are specified. Default: no.
dnssl local switchUse only local DNSSL definitions for this interface. See
rdnss localoption above. Default: no.
Prefix specific options
skip switchThis option allows to specify that given prefix should not be advertised. This is useful for making exceptions from a default policy of advertising all prefixes. Note that for withdrawing an already advertised prefix it is more useful to advertise it with zero valid lifetime. Default: no
onlink switchThis option specifies whether hosts may use the advertised prefix for onlink determination. Default: yes
autonomous switchThis option specifies whether hosts may use the advertised prefix for stateless autoconfiguration. Default: yes
valid lifetime expr [sensitive switch]This option specifies the time (in seconds) how long (after the receipt of RA) the prefix information is valid, i.e., autoconfigured IP addresses can be assigned and hosts with that IP addresses are considered directly reachable. 0 means the prefix is no longer valid. For
sensitiveoption, see trigger. Default: 86400 (1 day),sensitiveno.preferred lifetime expr [sensitive switch]This option specifies the time (in seconds) how long (after the receipt of RA) IP addresses generated from the prefix using stateless autoconfiguration remain preferred. For
sensitiveoption, see trigger. Default: 14400 (4 hours),sensitiveno.
RDNSS specific options:
ns addressThis option specifies one recursive DNS server. Can be used multiple times for multiple servers. It is mandatory to have at least one
nsoption inrdnssdefinition.lifetime [mult] exprThis option specifies the time how long the RDNSS information may be used by clients after the receipt of RA. It is expressed either in seconds or (when
multis used) in multiples ofmax ra interval. Note that RDNSS information is also invalidated whendefault lifetimeexpires. 0 means these addresses are no longer valid DNS servers. Default: 3 *max ra interval.
DNSSL specific options:
domain addressThis option specifies one DNS search domain. Can be used multiple times for multiple domains. It is mandatory to have at least one
domainoption indnssldefinition.lifetime [mult] exprThis option specifies the time how long the DNSSL information may be used by clients after the receipt of RA. Details are the same as for RDNSS
lifetimeoption above. Default: 3 *max ra interval.
Attributes
RAdv defines two route attributes:
enum ra_preferenceThe preference of the route. The value can be RA_PREF_LOW, RA_PREF_MEDIUM or RA_PREF_HIGH. If the attribute is not set, the route preference option is used.
int ra_lifetimeThe advertised lifetime of the route, in seconds. The special value of 0xffffffff represents infinity. If the attribute is not set, the route lifetime option is used.
Example
table radv_routes; # Manually configured routes go here
protocol static {
table radv_routes;
route 2001:0DB8:4000::/48 unreachable;
route 2001:0DB8:4010::/48 unreachable;
route 2001:0DB8:4020::/48 unreachable {
ra_preference = RA_PREF_HIGH;
ra_lifetime = 3600;
};
}
protocol radv {
propagate routes yes; # Propagate the routes from the radv_routes table
table radv_routes;
export all;
interface "eth2" {
max ra interval 5; # Fast failover with more routers
managed yes; # Using DHCPv6 on eth2
prefix ::/0 {
autonomous off; # So do not autoconfigure any IP
};
};
interface "eth*"; # No need for any other options
prefix 2001:0DB8:1234::/48 {
preferred lifetime 0; # Deprecated address range
};
prefix 2001:0DB8:2000::/48 {
autonomous off; # Do not autoconfigure
};
rdnss 2001:0DB8:1234::10; # Short form of RDNSS
rdnss {
lifetime mult 10;
ns 2001:0DB8:1234::11;
ns 2001:0DB8:1234::12;
};
dnssl {
lifetime 3600;
domain "abc.com";
domain "xyz.com";
};
}
6.11 RIP
Introduction
The RIP protocol (also sometimes called Rest In Pieces) is a simple protocol, where each router broadcasts (to all its neighbors) distances to all networks it can reach. When a router hears distance to another network, it increments it and broadcasts it back. Broadcasts are done in regular intervals. Therefore, if some network goes unreachable, routers keep telling each other that its distance is the original distance plus 1 (actually, plus interface metric, which is usually one). After some time, the distance reaches infinity (that's 15 in RIP) and all routers know that network is unreachable. RIP tries to minimize situations where counting to infinity is necessary, because it is slow. Due to infinity being 16, you can't use RIP on networks where maximal distance is higher than 15 hosts.
BIRD supports RIPv1 (RFC 1058), RIPv2 (RFC 2453), RIPng (RFC 2080), and RIP cryptographic authentication (RFC 4822).
RIP is a very simple protocol, and it has a lot of shortcomings. Slow convergence, big network load and inability to handle larger networks makes it pretty much obsolete. It is still usable on very small networks.
Configuration
RIP configuration consists mainly of common protocol options and interface definitions, most RIP options are interface specific.
protocol rip [<name>] {
infinity <number>;
ecmp <switch> [limit <number>];
interface <interface pattern> {
metric <number>;
mode multicast|broadcast;
passive <switch>;
address <ip>;
port <number>;
version 1|2;
split horizon <switch>;
poison reverse <switch>;
check zero <switch>;
update time <number>;
timeout time <number>;
garbage time <number>;
ecmp weight <number>;
ttl security <switch>; | tx only;
tx class|dscp <number>;
tx priority <number>;
rx buffer <number>;
tx length <number>;
check link <switch>;
authentication none|plaintext|cryptographic;
password "<text>";
password "<text>" {
id <num>;
generate from "<date>";
generate to "<date>";
accept from "<date>";
accept to "<date>";
from "<date>";
to "<date>";
algorithm ( keyed md5 | keyed sha1 | hmac sha1 | hmac sha256 | hmac sha384 | hmac sha512 );
};
};
}
infinity numberSelects the distance of infinity. Bigger values will make protocol convergence even slower. The default value is 16.
ecmp switch [limit number]This option specifies whether RIP is allowed to generate ECMP (equal-cost multipath) routes. Such routes are used when there are several directions to the destination, each with the same (computed) cost. This option also allows to specify a limit on maximum number of nexthops in one route. By default, ECMP is disabled. If enabled, default value of the limit is 16.
interface pattern [, ...] { options }Interface definitions specify a set of interfaces on which the protocol is activated and contain interface specific options. See interface common options for detailed description.
Interface specific options:
metric numThis option specifies the metric of the interface. When a route is received from the interface, its metric is increased by this value before further processing. Valid values are 1-255, but values higher than infinity has no further meaning. Default: 1.
mode multicast|broadcastThis option selects the mode for RIP to use on the interface. The default is multicast mode for RIPv2 and broadcast mode for RIPv1. RIPng always uses the multicast mode.
passive switchPassive interfaces receive routing updates but do not transmit any messages. Default: no.
address ipThis option specifies a destination address used for multicast or broadcast messages, the default is the official RIP (224.0.0.9) or RIPng (ff02::9) multicast address, or an appropriate broadcast address in the broadcast mode.
port numberThis option selects an UDP port to operate on, the default is the official RIP (520) or RIPng (521) port.
version 1|2This option selects the version of RIP used on the interface. For RIPv1, automatic subnet aggregation is not implemented, only classful network routes and host routes are propagated. Note that BIRD allows RIPv1 to be configured with features that are defined for RIPv2 only, like authentication or using multicast sockets. The default is RIPv2 for IPv4 RIP, the option is not supported for RIPng, as no further versions are defined.
version only switchRegardless of RIP version configured for the interface, BIRD accepts incoming packets of any RIP version. This option restrict accepted packets to the configured version. Default: no.
split horizon switchSplit horizon is a scheme for preventing routing loops. When split horizon is active, routes are not regularly propagated back to the interface from which they were received. They are either not propagated back at all (plain split horizon) or propagated back with an infinity metric (split horizon with poisoned reverse). Therefore, other routers on the interface will not consider the router as a part of an independent path to the destination of the route. Default: yes.
poison reverse switchWhen split horizon is active, this option specifies whether the poisoned reverse variant (propagating routes back with an infinity metric) is used. The poisoned reverse has some advantages in faster convergence, but uses more network traffic. Default: yes.
check zero switchReceived RIPv1 packets with non-zero values in reserved fields should be discarded. This option specifies whether the check is performed or such packets are just processed as usual. Default: yes.
update time numberSpecifies the number of seconds between periodic updates. A lower number will mean faster convergence but bigger network load. Default: 30.
timeout time numberSpecifies the time interval (in seconds) between the last received route announcement and the route expiration. After that, the network is considered unreachable, but still is propagated with infinity distance. Default: 180.
garbage time numberSpecifies the time interval (in seconds) between the route expiration and the removal of the unreachable network entry. The garbage interval, when a route with infinity metric is propagated, is used for both internal (after expiration) and external (after withdrawal) routes. Default: 120.
ecmp weight numberWhen ECMP (multipath) routes are allowed, this value specifies a relative weight used for nexthops going through the iface. Valid values are 1-256. Default value is 1.
authentication none|plaintext|cryptographicSelects authentication method to be used.
nonemeans that packets are not authenticated at all,plaintextmeans that a plaintext password is embedded into each packet, andcryptographicmeans that packets are authenticated using some cryptographic hash function selected by optionalgorithmfor each key. The default cryptographic algorithm for RIP keys is Keyed-MD5. If you set authentication to not-none, it is a good idea to addpasswordsection. Default: none.password "text"Specifies a password used for authentication. See password common option for detailed description.
ttl security [switch | tx only]TTL security is a feature that protects routing protocols from remote spoofed packets by using TTL 255 instead of TTL 1 for protocol packets destined to neighbors. Because TTL is decremented when packets are forwarded, it is non-trivial to spoof packets with TTL 255 from remote locations.
If this option is enabled, the router will send RIP packets with TTL 255 and drop received packets with TTL less than 255. If this option si set to
tx only, TTL 255 is used for sent packets, but is not checked for received packets. Such setting does not offer protection, but offers compatibility with neighbors regardless of whether they use ttl security.For RIPng, TTL security is a standard behavior (required by RFC 2080) and therefore default value is yes. For IPv4 RIP, default value is no.
tx class|dscp|priority numberThese options specify the ToS/DiffServ/Traffic class/Priority of the outgoing RIP packets. See tx class common option for detailed description.
rx buffer numberThis option specifies the size of buffers used for packet processing. The buffer size should be bigger than maximal size of received packets. The default value is 532 for IPv4 RIP and interface MTU value for RIPng.
tx length numberThis option specifies the maximum length of generated RIP packets. To avoid IP fragmentation, it should not exceed the interface MTU value. The default value is 532 for IPv4 RIP and interface MTU value for RIPng.
check link switchIf set, the hardware link state (as reported by OS) is taken into consideration. When the link disappears (e.g. an ethernet cable is unplugged), neighbors are immediately considered unreachable and all routes received from them are withdrawn. It is possible that some hardware drivers or platforms do not implement this feature. Default: no.
Attributes
RIP defines two route attributes:
int rip_metricRIP metric of the route (ranging from 0 to
infinity). When routes from different RIP instances are available and all of them have the same preference, BIRD prefers the route with lowestrip_metric. When a non-RIP route is exported to RIP, the default metric is 1.int rip_tagRIP route tag: a 16-bit number which can be used to carry additional information with the route (for example, an originating AS number in case of external routes). When a non-RIP route is exported to RIP, the default tag is 0.
Example
protocol rip {
import all;
export all;
interface "eth*" {
metric 2;
port 1520;
mode multicast;
update time 12;
timeout time 60;
authentication cryptographic;
password "secret" { algorithm hmac sha256; };
};
}
6.12 Static
The Static protocol doesn't communicate with other routers in the network, but instead it allows you to define routes manually. This is often used for specifying how to forward packets to parts of the network which don't use dynamic routing at all and also for defining sink routes (i.e., those telling to return packets as undeliverable if they are in your IP block, you don't have any specific destination for them and you don't want to send them out through the default route to prevent routing loops).
There are five types of static routes: `classical' routes telling to forward packets to a neighboring router, multipath routes specifying several (possibly weighted) neighboring routers, device routes specifying forwarding to hosts on a directly connected network, recursive routes computing their nexthops by doing route table lookups for a given IP, and special routes (sink, blackhole etc.) which specify a special action to be done instead of forwarding the packet.
When the particular destination is not available (the interface is down or the next hop of the route is not a neighbor at the moment), Static just uninstalls the route from the table it is connected to and adds it again as soon as the destination becomes adjacent again.
There are three classes of definitions in Static protocol configuration -- global options, static route definitions, and per-route options. Usually, the definition of the protocol contains mainly a list of static routes.
Global options:
check link switchIf set, hardware link states of network interfaces are taken into consideration. When link disappears (e.g. ethernet cable is unplugged), static routes directing to that interface are removed. It is possible that some hardware drivers or platforms do not implement this feature. Default: off.
igp table nameSpecifies a table that is used for route table lookups of recursive routes. Default: the same table as the protocol is connected to.
Route definitions (each may also contain a block of per-route options):
route prefix via ipStatic route through a neighboring router. For link-local next hops, interface can be specified as a part of the address (e.g.,
via fe80::1234%eth0).route prefix multipath via ip [weight num] [bfd switch] [via ...]Static multipath route. Contains several nexthops (gateways), possibly with their weights.
route prefix via "interface"Static device route through an interface to hosts on a directly connected network.
route prefix recursive ipStatic recursive route, its nexthop depends on a route table lookup for given IP address.
route prefix blackhole|unreachable|prohibitSpecial routes specifying to silently drop the packet, return it as unreachable or return it as administratively prohibited. First two targets are also known as
dropandreject.
Per-route options:
bfd switchThe Static protocol could use BFD protocol for next hop liveness detection. If enabled, a BFD session to the route next hop is created and the static route is BFD-controlled -- the static route is announced only if the next hop liveness is confirmed by BFD. If the BFD session fails, the static route is removed. Note that this is a bit different compared to other protocols, which may use BFD as an advisory mechanism for fast failure detection but ignores it if a BFD session is not even established.
This option can be used for static routes with a direct next hop, or also for for individual next hops in a static multipath route (see above). Note that BFD protocol also has to be configured, see BFD section for details. Default value is no.
filter expressionThis is a special option that allows filter expressions to be configured on per-route basis. Can be used multiple times. These expressions are evaluated when the route is originated, similarly to the import filter of the static protocol. This is especially useful for configuring route attributes, e.g.,
ospf_metric1 = 100;for a route that will be exported to the OSPF protocol.
Static routes have no specific attributes.
Example static config might look like this:
protocol static {
table testable; # Connect to a non-default routing table
check link; # Advertise routes only if link is up
route 0.0.0.0/0 via 198.51.100.130; # Default route
route 10.0.0.0/8 multipath # Multipath route
via 198.51.100.10 weight 2
via 198.51.100.20 bfd # BFD-controlled next hop
via 192.0.2.1;
route 203.0.113.0/24 unreachable; # Sink route
route 10.2.0.0/24 via "arc0"; # Secondary network
route 192.168.10.0/24 via 198.51.100.100 {
ospf_metric1 = 20; # Set extended attribute
}
route 192.168.10.0/24 via 198.51.100.100 {
ospf_metric2 = 100; # Set extended attribute
ospf_tag = 2; # Set extended attribute
bfd; # BFD-controlled route
}
}
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