[c-nsp] MTU and PMTUD

[Saku Ytti]

> Assuming typical MPLS network running L2VPNs and L3VPNs, how do you
> configure MTU values for core interfaces? Do you set interface (L2) MTU,
> MPLS MTU and IP MTU separately?

We set the 'physical' MTU (In IOS-XR+Junos L2 but no CRC is in this
humber) as high as it went when the decision was made. Today you can
do I think 10k in Cisco8k and 16k in Juniper. We do not seet MPLS or
IP MTUs separately in Core. On the edge you should always set L3 MTU,
because you want to have the ability to add subinterfaces with large
MTU, so physical MTU must be large, as change will affect all
subinterfaces. This way you can later add big MTU subint, without
affecting other subints.

> Do you enable PMTUD for TCP based control plane protocols like BGP or LDP?

No, not at this time, our BGP transfer performance is limited by TCP
window-size, so larger packets would not do anything for us.  And LDP
has a trivial amount of stable data so it doesn't matter.

-- 
  ++ytti