[j-nsp] BGP MTU Mismatch

[Ido Szargel]

Hi Keegan,

What is most likely to happen is that the session established since those are small packets, then the routers try to send the route messages which are large in case you are sharing full routing table, those messages do not arrive to the other side because of the MTU mismatch, thus causing the keepalive messages to be delayed as well, then when the dead interval kicks in the session flaps.
PMTU under the BGP group/neighbor should normally be enough to solve it.

Regards,
Ido.

-----Original Message-----
From: juniper-nsp-bounces at puck.nether.net [mailto:juniper-nsp-bounces at puck.nether.net] On Behalf Of Keegan Holley
Sent: Wednesday, June 22, 2011 11:38 AM
To: juniper-nsp
Subject: [j-nsp] BGP MTU Mismatch

Does anyone know why a BGP session would constantly flap because of an MTU mismatch.  I'm sure it's MTU since that is what fixed the problem.  The peering is between a cisco and a juniper and both support PMTU discovery.  I would assume any mismatches would be settled by the TCP MSS negotiation or fragmentation (admittedly bad).  The peering flapped almost every three minutes on the mark so it never made it past the first dead timer interval.
 Just curious if someone out there had ever gotten to the bottom of this problem.
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