Fwd: RE: [c-nsp] Number of MPLS labels and MTU

[Mark Lewis]

Hi,

>
>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.


Yep- just as you says, it depends on the location of the tunnel endpoints, whether FRR is configured, etc- and this is stated and a number of examples given in the book. And there's also a case study that describes the label stack of L3VPN over TE tunnels.


>
> > 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.

Agreed- you just need to work out the maximum number of labels in the stack, add this to the max IP packet size that you receive from customers (assuming L3VPN), and then set this on LSR i/fs using 'mpls mtu' (and config jumbo frame support on switches). If you're not sure of the label stack size for your particular config then it may be a good idea to err on the safe side and add enough 'headroom' for one or two 'extra' labels, if possible.

Also, note the relatively recent very slight modification to the use of the 'mpls mtu' command:

http://www.cisco.com/en/US/products/ps6566/products_feature_guide09186a00805b5519.html

Finally, if you are transporting L2 frames over your MPLS backbone then make sure you take into account the MPLS MTU required for these typically larger labelled packet sizes (L2 frame + control word + label stack). But that's covered in detail in chapter 7 :)

Cheers,

Mark