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

[Saku Ytti]

Hey James,

On 17 April 2018 at 11:25, James Bensley <jwbensley at gmail.com> wrote:

> 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

BGP messages are limited to 4096.

Having said that, OPs configuration is broken, and should be fixed.
MTU can only change inside router, not inside a link or a switch, it's
non-recoverable configuration error.

Best MTU practice:
a) configuration MTU on physical interface as large are reasonable
(you can choose some reasonable LCD, if that's large enough)
b) on logical interfaces, configure protocol MTU to what ever MTU
desired by far-end and supported by transport

Some MTU calculations for what is '1500B MTU'

L3 MTU == 1500B
L2 MTU == 1518B (6 + 6 + 2 + L3 + 4)
L1 MTU == 1538B (1 + 7 + L2 + 12)
JNPR MTU == 1514 (6 + 6 + 2 + L3)
JNPR ICMP == 1547 (L3 - 20 - 8)

All of these are same MTU, and people are using them exchangeably
without actually knowing if A and B end agree on the MTU. So not
rarely, but actually typically A and B ends on large MTU networks have
different MTU configured, when non-standard 1500B MTU is used, as A-B
parties are unable to communicate to each other what is the correct
number that needs to be configured.
Mostly this brokenness is not visible to anyone, because routers are
not offered large frames, but if they were, they'd be blackholed. It's
damn shame we didn't fix this win ARP/ND, as we seem unable to
configure MTU correctly manually.


-- 
  ++ytti