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

Chris Hale chale99 at gmail.com
Sun May 31 22:42:02 EDT 2009

On Sun, May 31, 2009 at 9:04 PM, Ray Burkholder <ray at oneunified.net> wrote:

> >
> >
> > 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.

I have an available DS3 interface on each of the POP H routers.  Maybe I
will set that up tomorrow and push the MPLS traffic across this interconnect
to see if that helps.  Maybe the mpls mtu setting on the PA-FE-TX interfaces
just isn't working.  I have also forced the GigE MPLS MTU settings on the
backbone link between the POPs to 1538 as they were at the default of 1500

Thanks again,

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