[c-nsp] MTU packet loss problem 12410 XR and 6509

[Lee Starnes]

Hello everyone,

A strange MTU issue has popped up and for the life of me I am unable to
figure out why. This seems to only affect one Metro-E carrier and only when
the traffic passes between the 6500 and the 12410.

ME Carrier A ---> 10G 6509 ----bundle-ether1(4G)--->12410A
ME Carrier B ---/                 \-----bundle-ether2(4G)--->12410B
ME Carrier C --/

Traffic that passes from either 12410 to customer links on ME carrier A are
seeing MTU issues and packet loss. Traffic across those same links for
carriers B and C have no issue. To test this, we can ping from from the
12410 to a site on ME carrier A with 1500byte packet size and get packet
loss. The same test to clients on ME carrier B and C have no issues. Now,
since no changes were made on our end and the carrier states no changes
were made on their end, we are at a standstill.

However, I did see that the MTU size on the 12410's is by default 1514 and
the MTU on the 6500 is 1500. Changing this to match 1500 on both sides
causes no traffic to pass. I'm not sure why both sides of the bundle-ether
interfaces matching MTU causes 100% packet loss.

Anybody have any ideas on why matching MTU size would cause no traffic to
pass? Ultimately the carrier will need to fix their issue, but I would like
to understand why this problem of matched MTU sizes causes no traffic.

Thanks.

-Lee