[j-nsp] J-Series to Cisco Multilink problem

[Phil Taylor]

Hi. 

This is my first post to this list so please be gentle with me :-)

I am trying to get a 3 x E1 multilink on a J4300 (running 7.2r1.7)
talking to a remote Cisco (I think it is a 7500 running 10.2(27)s4)

The connection is up and working but it appears to be negotiating an MTU
of 1496 with the remote router:

ls-0/0/0.0
Encapsulation:Multilink-PPPFamily:inet MTU:1496 Flags: None Route
Table:0 
Local Destination Broadcast Flags 
x.x.x.x x.x.x.x/30 Unspecified Is-PreferredIs-Primary 

If I try to force an MTU  on ls-0/0/0 of 1500 or anything higher it
still seems to show an MTU of 1496. 

The remote end are absolutely sure that they have an MTU of 1500 set but
any outbound traffic from my site with a packet size bigger than 1496 is
getting dropped which as you can imagine is pretty bad. I did wonder if
it is to do with Cisco not including tcp overhead in their MTU's.

If I force a client or server mtu of 1496 then everything works OK but I
can't really ask everyone to change their MTU !!!

I do have an open ticket but I wondered if anyone has any suggestions, I
will happilly forward my config to anyone that is interested.

Cheers

Phil

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