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

[Mike Benjamin]

Juniper and Cisco don't play nice with MRRU/MTU with MLPPP.

Cisco sets their MTU to 1500 and it is not configurable.

Juniper sets their MTU to 4 bytes below the MRRU and it is not
configurable.

Prior to 12.2something in IOS MRRU was not configurable, and was
set to 1524.

So, if you set the Juniper to MRRU 1524, Cisco defaults to MRRU 1524,
Juniper MTU is 1520, IOS MTU is 1500.  MTU mismatch.

The solution?  Make sure you're running an IOS with configurable MRRU
(it will actually negotiate off the Juniper value if you don't
configure it) and set the Juniper side to an MRRU of 1504.

That will result in MRRU = 1504 and MTU = 1500 on both boxes.

Quite the pain..  Unless someone else knows as a way around this?
Or I am just missing something..

--mikeb

On Thu, May 05, 2005 at 12:26:10PM +0100, Phil Taylor wrote:
: 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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--               
Mike Benjamin   =   mikeb at mikeb.org