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

[Justin M. Streiner]

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