[c-nsp] MTU on GigE

[Chad Skidmore]

In my experience it depends on the device and interface you are working
with.  There is likely a "secret decoder wheel" somewhere at Cisco but
so far I have not been able to find it.  On GSRs with 1-port GE line
cards you have to increase the interface MTU and then set IP MTU to
1500.  The MPLS MTU is then assumed to be at whatever the interface MTU
is at.  On 7500s and 7200s you leave the main interface MTU alone and
set MPLS MTU to a higher setting (1546 works well for me).  On 3750
metros it is dependant on what IOS REV you run.  The TAC told me that
the config I wanted to run would not work and they were correct for one
IOS rev but not a different one.

YMMV depending on your combo of IOS rev, device, and interface type.

Good luck,
Chad
 

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Phillip Vandry
Sent: Sunday, November 06, 2005 3:07 PM
To: Vincent De Keyzer
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] MTU on GigE

On Wed, Oct 19, 2005 at 02:54:47PM +0200, Vincent De Keyzer wrote:
> So you have a "physical" MTU, and a MTU for each protocol; and no
"protocol"
> MTU can be higher than the "physical" MTU.

Exception: "mpls mtu" (or "tag-switching mtu") can be larger than the
physical interface MTU... for some reason.

-Phil
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