[c-nsp] ip tcp adjust-mss

[Mack McBride]

mtu = Ethernet MTU
ip mtu = MTU used for IP packets originating on the box.
mpls mtu = maximum MTU for MPLS encapsulated packets

Some boxes also have other MTU commands.

LR Mack McBride
Network Architect

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Eric A Louie
Sent: Monday, February 11, 2013 1:59 PM
To: Saku Ytti; cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] ip tcp adjust-mss

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 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.

--
  ++ytti
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