[c-nsp] ip tcp adjust-mss

Eric A Louie elouie at yahoo.com
Mon Feb 11 15:58:42 EST 2013

Ok, maybe I'm missing the obvious, but within my backbone, I can't just increase 
the MTU across the Ethernet links.

router (config-if)#ip mtu ?
  <68-1500>  MTU (bytes)

Unless this is the mtu you refer to
router (config-if)#mtu ?

  <1500-9800>  MTU size in bytes
 Much appreciated, Eric

From: Saku Ytti <saku at ytti.fi>
To: cisco-nsp at puck.nether.net
Sent: Mon, February 11, 2013 12:33:53 PM
Subject: Re: [c-nsp] ip tcp adjust-mss

On (2013-02-11 11:56 -0800), Eric A Louie wrote:

> Is anyone else using this method of "mtu control"?  I need some support - my 
> is asking why I have to do this, and who else does it, and is it a common 
> practice, etc, so I'm looking for evidence, more than just "The Cisco TAC told 

> me to do it".

Very common hack to deal when tunneling is involved in middle of the
network, and reducing client MTU is not practical. But I'm really surprised
you'd need it in this situation, usually you can increase your core MTU to
carry MPLS labels while still delivering customers 1500B.

Mostly while quite ugly hack, it just works. Sometimes you run into some
poor application which send MTU size UDP frames and expect them to be
delivered, those customers would not be happy.

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