[c-nsp] MTU size with EoMPLS

[Peter Rathlev]

Hi Jose,

You need to enable large MTU on the core facing interfaces on both
sides, and on all interfaces on the core routers. The interfaces that
have LDP enabled is no bad guess.

You can raise the interface MTU with the "mtu X" interface config
command. If you don't specify any "mpls mtu X" command it will follow
the interface MTU. Don't adjust "mpls mtu" unless you have a good
reason.

You would need 1518 bytes Ethernet + 4 bytes 802.1Q (if that's your
protocol) and 4+ bytes MPLS (depending on how many labels you stack).
I'm not sure the 1811 supports baby giants/jumbo frames, but you can try
at least. I've seen a value of 1546 used often, although I don't know
exactly why. :-)

Regards,
Peter


On Fri, 2008-02-29 at 13:21 -0500, The Father wrote:
> Hi group.  I'm wondering if someone can help me out with a bit of an MTU 
> issue I'm seeing when playing around with EoMPLS in our lab.
> 
> This is the topology I have setup:
> 
> 3550-24------3750-----7603/Sup32-------1811-------7206VXR-NPE300-----3750------3550-24 
> 
> 
> LDP is enabled between the 7600, 1811 and 7200 (yes it's an 1811 as my P 
> for now).  I have created VLAN 800 on the first 3550 and used the 
> xconnect command on the ingress Gig interface of the 7603 (sub-interface 
> is used).  The circuit is encapsulated and sent over to the 7206VXR 
> (sub-interface used here too).  From there it is sent over to the other 
> 3550 (obviously trunking is being done between the 3550s and 3750s).  We 
> have also created SVIs on the 3550s to act as our layer 3 interfaces 
> with IPs.
> 
> When we try and ping from one end to the other with 1500 byte sized 
> packets it fails.  The highest we can go is 1478.  I know that with MPLS 
> you normally are required to enable jumbo frame support but where 
> exactly would I do this?  Is it on the interfaces that have LDP enabled 
> on them or in the entire network I have setup above?  Currently 
> everything is set to the default of 1500.
> 
> Another thing that I'm confused about is when to use the "mpls mtu xxx" 
> command.  If I understand it correctly, you must have the interface MTU 
> set higher than the MPLS MTU.  I have tried using the "mpls mtu 1520" 
> command on the LDP enabled interfaces but it has made no 
> difference...probably because the interface MTU is still 1500.
> 
> Any thoughts or advice on trying to resolve this?
> 
> Thanks.
> 
> Jose
> 
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