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

Mark Tinka mark.tinka at seacom.mu
Sun Oct 27 12:01:48 EDT 2013

On Sunday, October 27, 2013 02:02:08 PM Xu Hu wrote:

> any particular reason for this?
> I meant you set 9192 & 9178? did you consider the mpls
> stack labeles or Layer 2 headers? or whatever, then
> finally you made this recommendation value?

On Ethernet interfaces, IOS and IOS XE automatically account 
for the 14-byte Layer 2 Ethernet overhead when you set an 
MTU value.

Junos and IOS XR do not, and any MTU value defined is 
considered, by the router or switch, to be the "true" MTU of 
the interface.

So whatever you set on Junos and IOS XR system, subtract 14 
bytes (for Ethernet) for IOS and IOS XE systems and apply 

Also, at this MTU level, you should have no issues 
supporting various MPLS application, be it simple IP 
encapsulation, or a label stack that signals advanced 
services like MPLS-based VPNS, BGP-MVPN's, MPLS-TE, e.t.c.

If your customers were requesting for Jumbo frames with 
their (on-net) service, they should be fine too. We normally 
guarantee 9,000 bytes to customers using this for on-net 
traffic, and offer no guarantees for anything higher (even 
if, technically, we can support it edge-to-edge).


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