OSPF: problem with route insert and delete sequences

Gerold Gruber gruber at citkomm.de
Thu Sep 25 12:49:48 CEST 2014

Hi list,

we built a network with 2 OSPF areas and
18 Linux based Bird OSPF routers, running Bird 1.4.4 on Debian wheezy
And there are even more routers from other vendors.

Part of this network is a VPN gateway which also takes part
in OSPF routing.

On this gateway up- and down scripts set or delete routes
to the connected sites. So these routes are set or deleted in the kernel
routing table and then noticed by Bird and ditributed.

Now comes the problem:
if one particular route is repeatedly set and deleted within a few
seconds (as it appears when a VPN line flaps), then we see the following:
Bird propagates each change to his neighbours immediately, but stops
doing so after a few changes (3 to 5), at least the neighbour does not
show the changes in his debug log anymore.

This can be reproduced by setting and deleting kernel routes on the
command line, with sleep 1 between.

A Cisco box apparently does some kind of filtering: each change is
propagated after ~5 seconds if no further change within this interval
negates the change. (e.g.: route is deleted and re-inserted within 1
second, then no change is propagated to the neighbours).

As Bird stops propagating these changes it is some kind of random which
state the neighbours are left. So wrong routes may remain (delete is
missing at the end), or reappearing routes are not propagated and lost.

So from the behaviour seen with the current release some kind of serious
problem arises for us.
Probably this is related to "future work" Route aggregation and flap
dampening from the Bird web site?

What would be a workaround to circumvent this?
Are there plans to fix this in the near future?

Best regards

Abteilung Systembetrieb
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E-Mail:         gruber at citkomm.de
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