[c-nsp] strange behavior over MPLS network - remote desktopwon't work

Ray Burkholder ray at oneunified.net
Sun May 31 21:04:36 EDT 2009

> What does that indicate to you?  1472 + VLAN tag plus MPLS < 1500?

When provisioning MPLS circuits, one has to be careful.  Basic MPLS will
attach one or more 4 byte labels on to each packet.  Psuedowires attach
additional bytes onto each packet.  WAN circuits running MPLS need to be
provisioned such that the interface MTU is set to 1500 PLUS any pseudowire
overhead plus any MPLS label overhead.  If you try to run MPLS stuff across
a standard 1500 MTU WAN interface, you get the problems you are now
encountering: fragmentation, drops, corruption, ...  Some protocols can
handle it, but I've read that RDP sets the no-fragment bit, thus dropping
the packets.

STM-1 and DS3 circuits run by default at 4470 bytes so easily accommodate
MPLS overhead.  Ethernet circuits are at 1500, and you have to work with
upstream vendors to ensure their networks can handle MTU's greater than
1500.  Cisco switches need a reboot after setting a system mtu setting.
Routers can change interface mtu settings on the fly.

You could try setting your MTU setting on your pc to 1300 and see if things
work.  If they do, then you know you have an upstream mtu problem.


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