[c-nsp] ip tcp adjust-mss

Paul Stewart paul at paulstewart.org
Mon Feb 11 15:43:52 EST 2013

We used to use "ip tcp adjust-mss" a lot for applications where PPPOE was
required..... ie Cisco 877 router at customer premise connecting via PPPOE
back to us.

Never seen it in MPLS core functions though if that's where it's being


-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Saku Ytti
Sent: February-11-13 3:23 PM
To: cisco-nsp at puck.nether.net
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 CEO 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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