OSPF type broadcast over GRE for NRHP

[Ondrej Zajicek]

On Mon, Mar 02, 2015 at 03:49:48PM -0500, Clint Armstrong wrote:
> I am trying to use BIRD to run OSPF inside a Phase 3 DMVPN using OpenNHRP.
> The problem I'm seeing is that it appears to be impossible to force type
> broadcast on a GRE interface. When I try, the message "Cannot use interface
> gre1 as broadcast, forcing nbma" is logged.
> 
> This may normally make sense, but the OpenNHRP daemon captures multicast
> traffic on it's interface and forwards it appropriately.
> 
> A configuration parameter to bypass this protection and allow broadcast on
> a GRE interface would be very helpful.

Hi

Kernel reports whether the interface supports multicast and BIRD checks
it to see whether it can use modes using multicast on that iface
(OSPF 'broadcast' mode in reality uses multicast).

You could disable the check by commenting out these lines in proto/ospf/iface.c:

  if ((ifa->type == OSPF_IT_BCAST) && !(iface->flags & if_multi_flag))
    ifa->type = OSPF_IT_NBMA;


But perhaps the simpler way is to fix the value reported by kernel:

  ip link set greXXX multicast on


After that, BIRD accepts the interface as multicast and OSPF broadcast mode
can be used.

ip a a:

5: gre_x at NONE: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN group default 


birdc show interfaces:

gre_x up (index=5)
    PtP Multicast AdminUp LinkUp MTU=1476


I don't know about DMVPN / OpenNHRP, but for regular PtP GRE (and other)
tunnels it works without problems (packets are just send to the other
side). Linux kernel should report multicast flag for PtP GRE tunnels,
not reporting it is IMHO a long-term bug in Linux kernel.


-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
"To err is human -- to blame it on a computer is even more so."
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