Issues with SNMP monitoring over a GRE tunnel.

Jeff Walter jwalter at
Wed Nov 5 18:36:43 UTC 2014

I think the simple solution here is to query for fewer OIDs to get the
packet size (in both directions) down below the MTU. It'll take more
requests and thus longer, but if that's what solves the problem... well,
that's what solves the problem.

On Wed, Nov 5, 2014 at 7:59 AM, Brian Christopher Raaen <
mailing-lists at> wrote:

> I have two different customers where I am unable to monitor their networks
> due to GRE MTU issues.  This is monitoring cable modems so I can't change
> the MTU of the end device.  The problem I am having is that the modems are
> producing frames that appear to be larger than some kind of MTU limit in
> the system (we do not control the customer routers in either case).  One
> that I am looking at is dropping anything larger than 1472, and I have let
> to tune down on the other one.  In one case the customer endpoint is a
> Cisco ASR1K router and the other is a ASR9K.  because these are UDP packets
> I can't use a mss to clamp things down.  Also I have been unable to
> replicate the issue in my lab, so I can't send them a list of commands to
> help fix the issue on their end.
> --
> Brian Christopher Raaen
> Network Architect
> Zcorum

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