[c-nsp] mpls vpn with MTU less than 1500

Lasher, Donn DLasher at newedgenetworks.com
Wed Oct 10 13:06:01 EDT 2007

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Soon Kian
Sent: Tuesday, October 09, 2007 11:02 PM
Subject: [c-nsp] mpls vpn with MTU less than 1500

Hi Guys,

Wondering if anyone is providing your customers with mpls vpn with MTU less
than 1500, due to physical media constraint. i.e equipment unable to support
Jumbo frame. By means of running tcp-adjust mss etc. If yes, what's your
customer's expectation on this.


While it can be done technically, I personally wouldn't recommend it.

There are too many internet sites that block all ICMP, which kills Path MTU
Discovery, which blackholes customer traffic with any MTU larger than your
'squeeze' point. I've spent many hours fighting tcp-adjust, clearing the DF
bit, etc, and in the end, just gave in to fixing the equipment in the middle
so I didn't have to squeeze the MTU, or force every individual customer to
change their MTU to support my link.

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