[c-nsp] MTU size with EoMPLS

[The Father]

Peter Rathlev wrote:
> 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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>
>
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>   
Thanks for the tips Peter.  Is "ip mtu xxx" on an interface the same as 
using "mtu xxx"?  Seems like I don't have the ability to change the mtu 
on some of the linecards of the 7600.

Jose