[c-nsp] Best practice, MPLS and MTU settings

Mark Tinka mark.tinka at seacom.mu
Sat Oct 26 11:02:20 EDT 2013

On Friday, October 25, 2013 04:39:57 AM Nick Hilliard wrote:

> I aim towards a 9100 byte ip mtu, in order to guarantee a
> 9000 byte MTU for customers with plenty of overhead on
> our side.  Sometimes this isn't possible due to third
> party provider core link restrictions.

We are a Cisco and Juniper house.

To match things, we run 9,192 bytes on Juniper and IOS XR-
based Cisco platforms, as well as 9,178 bytes on IOS and IOS 
XE-based Cisco systems.

We have noted that some Juniper SDH/SONET line cards require 
you to set the MTU at the address family level (inet, inet6, 
iso and mpls) for the MTU to take effect. You'd normally 
apply it at the global interface level and each address 
family would infer what it needs, but not on SDH/SONET line 
cards. This issue does not affect Ethernet line cards.

Cisco FE ports tend to not support Jumbo frames (except 
SPA's on the XR 12000 and CRS systems), but Juniper have no 
issue supporting Jumbo frames on FE ports. Suffice it to 
say, I haven't used FE ports anywhere for nearly 5x years 

Several Cisco switches will top out at 9,000 bytes. Not much 
you can do about that if you have them. However, newer 
platforms shipping now do up that to be inline with what we 
prefer now.

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