[j-nsp] mx960 to mx960 via ciena 6500 - mtu smaller in the middle

[Luis Balbinot]

> This issue is my turning up new MX960's that are simply connected together with Ciena 6500 DWDM.... for me to have an MTU issue via DWDM is actually a surprise to me.  I pretty much always envisioned wave/lamda dwdm as darn near like having an actual fiber cable... no, not the case apparently... I'm surprised that the ciena dwdm service in this case is actually imposing an Ethernet mtu limitation !  wow, the more I think on it, the more I'm surprised about it... previously my older asr9k 15-node mpls ring was via fujitsu flashwave dwdm and I don't recall this occurring

DWDM is kind of agnostic in the core but for packet networks there are
OTU frames (max size = 16,320 bytes) and there is also the interface
line card that imposes the actual MTU for the Ethernet interface (9600
is a well-known value for DWDM networks). If you have, say, a 100G
transponder and a 10x10G line card then you have to frame all those
ports, they won't become light as 10G channels. Even 10G channels on a
regular 10G non-coherent network must be framed for FEC and everything
else.

If you have a colored interface directly from your router to the DWDM
system then you might skip that limitation (that depends on the
platform and linecard configuration).

Luis