Had the same problem a few months ago prepping my bkbn for a higher mtu support and noticed one bgp peer flapping, it was the same issue as described by others, mtu mismatch.
I'm assuming that the initial bgp buildup happens ok due to the bgp-pkt-size < supported mtu then once the update pkts get sent the pkt size re-negotiated mtu size (new pkt) and the lower of the two ends mtu gets honored. Now this wouldn't cause issues if it was cisco to cisco or juno to juniper as each follow and honor their own principles regarding what makes up the mtu value. However when your going cisco/juniper there is always a mismatch as juniper does not take into accnt the 4byes required for crc so fragmentation happens and if the pkt has DF set (as it should since max mtu negotiated on is suppose to be safe) the other side well drop the pkt and thus killing the bgp update pks and eventually timing out. Is why I set the juniper side +14 bytes to ensure mtu problems done come up (+18 on links doing dot1q) This is just what I think so suggestions and corrections are always welcomed Cheers Payam On 6/22/11, Alex <[email protected]> wrote: > If no one replies back with ICMP Unreach type 3 code 4 with "Next-hop MTU" > then yes > http://www.faqs.org/rfcs/rfc1191.html > Cheers > Alex > ----- Original Message ----- > From: Keegan Holley > To: Alex > Cc: Ido Szargel ; juniper-nsp > Sent: Wednesday, June 22, 2011 3:27 PM > Subject: Re: [j-nsp] BGP MTU Mismatch > > > I don't think this is correct. PMTU discovery uses probes with the DF bit > set, the largest one to make it end to end is assumed to be the MTU. > > > http://en.wikipedia.org/wiki/Path_MTU_Discovery > > > > 2011/6/22 Alex <[email protected]> > > PMTU relies on someone in the path telling you what the MTU is. > And there is no way to check if this someone is telling the truth :-( > I saw this in complex environments such as BGP over GRE over IPSec > tunnel. > In this case, the sp-* interface carrying GRE tunnel has to have > mtu-discovery itself enabled. > Otherwise it will assume 9192 bytes. > Cheers > Alex > > > ----- Original Message ----- From: "Ido Szargel" <[email protected]> > To: "Keegan Holley" <[email protected]>; "juniper-nsp" > <[email protected]> > Sent: Wednesday, June 22, 2011 9:50 AM > Subject: Re: [j-nsp] BGP MTU Mismatch > > > > > 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: [email protected] > [mailto:[email protected]] 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. > _______________________________________________ > juniper-nsp mailing list [email protected] > https://puck.nether.net/mailman/listinfo/juniper-nsp > > _______________________________________________ > juniper-nsp mailing list [email protected] > https://puck.nether.net/mailman/listinfo/juniper-nsp > > > > _______________________________________________ > juniper-nsp mailing list [email protected] > https://puck.nether.net/mailman/listinfo/juniper-nsp > > > > _______________________________________________ > juniper-nsp mailing list [email protected] > https://puck.nether.net/mailman/listinfo/juniper-nsp > -- Sent from my mobile device Payam Tarverdyan Chychi Network Security Specialist / Network Engineer _______________________________________________ juniper-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/juniper-nsp
