on-interface routes

Ondrej Zajicek santiago at crfreenet.org
Mon Sep 17 21:04:07 CEST 2012

On Mon, Sep 17, 2012 at 08:05:56PM +0400, Alexander V. Chernikov wrote:
>>> This workaround still has problems in case of direct* patterns
>>> reconfiguration.
>> What exact problem is in direct* pattern reconfiguration?
>> I guess there is a problem if there is some lower priority route with
>> next hop, which is blocked by higher priority device route from direct
>> protocol, which disappears for a while during reconfiguration. Is
>> this what you mean?
> Yes. Typically all interfaces running OSPF suffers from this problem.
>> But just standalone direct protocol reconfiguration shouldn't cause
>> problems (device route removal is blocked by kernel protocol).
> I haven't investigated this in details (despite I have several different  
> configurations where this problem appears from time to time).
> I'll try to do this.

My point is that the problem could appear only if BOTH these conditions

1) direct protocol is reconfiguring

2) OSPF[*] generates some non-direct route to directly connected ifaces
(e.g. your router is not active for that net while another is, or your
router loses link and check link is active. Or simply you have asymetric

[*] or some other protocol, but OSPF is typical.

>>> Can we update Direct protocol documentation to reflect this problem?
>> You mean that routes disappear for a moment during reconfiguration,
>> so if they are used to protect kernel routes they should not be changed?
> Not exactly.
> Currently doc states ", in most cases it is not needed to have these  
> device routes in BIRD routing table and to use the direct protocol.".
> I'm talking about adding something like
> The question is whether it is a good idea to have such device routes in  
> BIRD routing table.
> "You definitely want to add device routes  if you're running *BSD. This  
> is workaround for interface routes being removed when IGP route for the  
> same network flaps."
> s/OS/Linux/ kernel usually handles device routes for directly connected  
> networks by itself so we don't need


BTW, the last sentence is not correct - for Linux it works because
kernel protocol is currently written in such a way that it cannot
overwrite non-BIRD route in kernel. Generally it is possible to
overwrite these device routes and in (much) older versions of BIRD such
problems happened in Linux too.

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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