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

[James Bensley]

On 16 April 2018 at 20:58, Aaron Gould <aaron1 at gvtc.com> 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.

Hi Aaron,

I think MTU consistency is important. It will make for a support
issues if the two devices has 16,000 byte MTU configured and that
can't actually be achieved. Will the Level1 support/NOC guys know to
check the Ciene device too and will they be able to work out the
problem? You don't issue being escalated for what is the "normal"
behavior.

Also you say you have OSPF and LDP up but if you bring up BGP over
this link you may have issues. BGP packs UPDATE messages up to the TCP
MSS (derived from the link MTU). If you are carrying the full table
for example, then you could end up with BGP UPDATE messages 16000
bytes long they won't cross the link. You end up with BGP establishing
and after $timeout flapping because no UPDATES were received and this
process loops round indefinitely until you bodge the TCP MSS, use
PMTUD or (preferred choice) correct the MTU issue.

Another issue you may face is that it's best to have consistent MTUs
across you entire network, not just this point-to-point link. If other
parts of your network don't have this size MTU you may have some
hard-to-debug issues further down the road when a customer joins to
two different parts of the network with disparate MTU sizes.

Cheers,
James.