<Keepalives are temporarily in throttle due to closed TCP window>

Brian Dickson Brian.Dickson at concertia.com
Tue Sep 15 20:39:33 UTC 2009

And more specifically, possibly an interface MTU (or ip mtu, I forget which).

If there is a mismatch between ends of a link, in one direction, MTU-sized packets get sent, and the other end sees those as "giants".

I've seen situations where the MTU is calculated incorrectly, when using some technology that adds a few bytes (e.g. VLAN tags, MPLS tags, etc.).

On Cisco boxes, when talking to other Cisco boxes, even.

Take a look at the interfaces over which the peering session runs, at both ends.
I.e., is this the only BGP session *over that interface*, for the local box?

(It might not be the end you think it's at, BTW.)

Oh, and if you find something, please, let us know.
War stories make for great bar BOFs at NANOG meetings. :-)


-----Original Message-----
From: Mikael Abrahamsson [mailto:swmike at swm.pp.se] 
Sent: September-14-09 2:39 PM
To: Michael Ruiz
Cc: nanog at nanog.org
Subject: Re: <Keepalives are temporarily in throttle due to closed TCP window>

On Mon, 14 Sep 2009, Michael Ruiz wrote:

> I am having difficulty maintaining my BGP session from my 6509 with
> Sup-7203bxls to a 7206 VXR NPE-400.  The session bounces every 3
> minutes.  I do have other IBGP sessions that are established with no
> problems, however, this is the only IBGP peer that is bouncing
> regularly.
> What does exactly the message mean and how do I stabilize this?  Any
> help will be appreciated.

This is most likely an MTU problem. Your SYN/SYN+ACK goes thru, but then 
the first fullsize MSS packet is sent, and it's not getting to the 
destination. 3 minutes is the dead timer for keepalives, which are not 
getting thru either because of the stalled TCP session.

Mikael Abrahamsson    email: swmike at swm.pp.se

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