[c-nsp] Number of MPLS labels and MTU

Oliver Boehmer (oboehmer) oboehmer at cisco.com
Sun Feb 5 03:41:42 EST 2006

gladston at br.ibm.com <> wrote on Sunday, February 05, 2006 4:17 AM:

> Hi,
> Reading Troubleshooting Virtual Private Networks, by Mark Lewis, he
> states that if using VPN MPLS + TE, a label stack depth of three is
> used  (RSVP signaled TE label + TDP/LDP signaled IGP label + VPN
> label). 

Right, but this also depends on where the tunnel is (for a PE-PE tunnel,
you might not have an IGP label), and if you are using TE Fast-Reroute,
you could have an addtl. label during the backup case.

> Considering an application generates a packets with higher number of
> bytes (1472) and DF bit set, the packet will have 1514 bytes without
> label and 1514 + 4 + 4 +4 bytes travelling through the network. Do
> you agree? 

How do you arrive at 1514? 

> Would it be enough to configure jumbo frame support and mpls mtu
> under the physical interface or it would be necessary any command
> under the TE tunnel interface?

You need to make sure your physical interfaces will be able to transport
the maximum size (i.e. Payload + 12 (or 16) bytes MPLS shim header).
Setting the MTU on the TE Tunnel interface is not an option (I think)
since you might also sending tagged packets over the tunnel which can't
be fragmented. 


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