[c-nsp] BGP neighbor not establishing session

[Eric Oosting]

On Tue, Nov 23, 2010 at 3:00 PM, Lobo <lobotiger at gmail.com> wrote:

>
>
> On 11/23/2010 6:14 AM, Mark Tinka wrote:
>
>> Can't think of why this would be an issue.
>> The longest distance we have today between two iBGP
>> neighbors is 160ms (and soon, the farthest we'll have will
>> be about 230ms), and that has no problems at all.
>>
>> I'd suspect MTU issues here.
>>
>> Cheers,
>>
>> Mark.
>>
>
> It's looking like it somehow might be related to that.  For a test, we set
> the mtu on the toro-router2's interfaces back to 1500 from 9216.  After that
> was done, the sessions to the vanc routers both came up!  Strange because we
> didn't change any of the mtus on the vanc routers.....they're all still
> 9216.
>
> One thing I didn't mention before was that all of the interfaces on these
> routers had jumbo frames turned on.  The cloud between toro and vanc though
> is mostly limited to 1546 but this has never posed a problem before with the
> previous IOS versions.
>

The default MSS in older IOS was something in the 500s, so even if you had a
large MTU on the interface and the path would only accommodate 1500ish you'd
still be OK. It could be that your newer IOS either went to a larger default
MSS that could take advantage of the 9216 MTU and there was no PMTUD, or
PMTUD is busted in between so the MTU squeeze between the two routers isn't
detected.

-e


>
> Jose
>
> P.S. I've opened up a TAC case as well to see if they can figure something
> out.
>
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