PMTU and Broken Servers

Richard A Steenbergen ras at
Thu May 8 17:17:58 UTC 2003

On Thu, May 08, 2003 at 07:04:18PM +0200, Mikael Abrahamsson wrote:
> On Thu, 8 May 2003, Leo Bicknell wrote:
> > Could you define newer?  My limited testing with 12.0S and 12.2something
> > shows both returing MTU can't be set on a tunnel interface.
> You cannot change the MTU of the tunnel, but you can change the "ip mtu" 
> of the tunnel. I've done this on 7200 and 1600 that are running IOSes that 
> are at least a year old. The feature is at least 2-3 years old, because 
> that's the first time I encountered it.
> The router will do re-assembly on the tunnel level to be able to transfer 
> a 1500 byte sized IP packet non-fragmented.

Be warned, while fragmentation follows some form of fast path, reassembly
is process switched. A beefy 7206 will fragment 300Mbps easily, but will
reassemble around 30Mbps.

Richard A Steenbergen <ras at>
GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)

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