[j-nsp] BGP MTU Mismatch

[Alex]

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" <ido at oasis-tech.net>
To: "Keegan Holley" <keegan.holley at sungard.com>; "juniper-nsp" 
<juniper-nsp at puck.nether.net>
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: 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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