[c-nsp] mpls mtu of 1492

[Oliver Boehmer (oboehmer)]

> 
> I have a test platform of routers connected  to  Cisco 2950T switches.
>  I am testing mpls and mpls based vpns but unfortunately I have ran
> into problems. The 2950T do not support baby giants (frames)
> 
>
http://www.cisco.com/en/US/products/hw/switches/ps700/products_configura
tion_example09186a008010edab.shtml#subtopic1G
> 
> What will be the effect of setting the mpls mtu to a value lower than
> 1500. 

Well, you will obviously start to fragment large packets which need to
be labelled, something which should be avoided given the usual problems
with fragmentation/PMTUD and broken FW.
But if you absolutely can't use kit with higher MTU support, you want to
configure the lower MPLS MTU closest to the edge as possible (even if
this interface supports a higher MTU) to avoid fragmenting in the core.

	oli