[c-nsp] MPLS MTU / Jumbo frames etc.

[Mikael Abrahamsson]

On Thu, 23 Jul 2009, Ray Burkholder wrote:

> When people define these MTU sizes, what does this size include?  The 
> payload?  The ip header?  Layer 2 header?  Some documentation seems 
> murky on this issue.

Depends on the platform. Several networks I have been working on has been 
standardised to 4470 IP MTU because this is a well known figure that most 
platforms and protocols support. This means that platforms which set L2 
MTU is set to 4484 for ethernet and 4474 for HDLC.

> When working with MTU changes necessary for MPLS operation, things get
> somewhat confusing.  For example in this document, somehow MPLS can have an
> MTU setting greater than what is allowed on the interface itself.

Several platforms do not do IP themselves well with jumboframes, but they 
can forward frames, and there is the interaction with L2 switches 
which is administrated thru the dist lan.

For these I recommend:

mtu 1546 (or something else)
ip mtu 1500
clns mtu 1497 (if you run isis).

For a 7200 with FE ports this translates into:

mpls mtu 1546

Please see discussion regarding this from ~1 year back.

-- 
Mikael Abrahamsson    email: swmike at swm.pp.se