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.
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