On Mon, 16 Apr 2018, Aaron Gould wrote:

See juniper interface MTU is set to max 16000 bytes. but when I ping I can
only get 9584 bytes through to the other side of the link.  This mx960 is
linked to another mx960, but Ciena 6500 dwdm is in between the mx960's.

Is there any reason why I should be concerned that currently Ciena tells us
that they only support 9600 byte MTU and that I have 16000 set on my MX960's
?

Is there an issue that could arise with this scenario ?

You're better off staying within the limits of what the intermediate gear will support. Fragmentation and re-assembly if you expect to fill a 16000 byte frame (or fill one enough to cause fragmentation as the frame crosses the Ciena gear) could impose a significant performance penalty.

It could also create problems that will be fun to debug if any of that gear starts dropping >MTU frames (think: code upgrades changing default behavior) rather than fragmenting them, not handling PMTUD properly, or not handling the fragments properly, or someone sets the DF
bit...

Currently ospf and ldp are neighboring fine and advertising routes and
labels

They're probably either not filling a 16000 byte frame, or fragmentation/re-assembly works properly.

You can always ramp up the MTU later after Ciena supports it.

jms
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