[j-nsp] Juniper MTU Math / eBGP multihop problem

[Paul Stewart]

Hi there..

 

Trying to understand some Juniper MTU related issues we're having.

 

ERX-310 router with dot1q AE connection to a pair of EX4200 switches.

 

AE interface on the ERX-310 is showing MTU of 1522 which using my "bad MTU
math" would be correct (1500+21 bytes overhead).

AE interface on the EX4200 switch is showing MTU of 1514 which again using
my bad math T is too low with dot1q overhead.

 

On one of the EX4200 switches is a copper GigE connection going out to an
ISP.  MTU on this port is 1514.

 

The problem is with BGP.  There are two sessions towards the ISP.  The first
is eBGP and works fine - BGP session comes up and stays stable.  The second
session though is eBGP multihop (about 5-6 hops away) and tears down after
90 seconds.

 

This is related to another message I posted recently to the list - thought
we had this figured out but nope. still having an issue. All we know at this
moment is the ISP is using Cisco equipment with an MTU of 1500 - pretty
vague.

 

So, with the above default MTU issues should anything really need to be
adjusted considering the first session works fine?  The ISP is more than
happy to investigate but I wanted to understand the MTU differences on the
Juniper equipment first.

 

Thanks for any input. I hate MTU issues ;)

 

Cheers,

 

Paul