How to set the mpls mtu and ip mtu ?

Unanswered Question
Jun 25th, 2008

The command "mpls mtu " is used to set the MPLS MTU value higher than the interface MTU value .

But I also see the note " Although you can set the MPLS MTU to a value greater than the interface MTU, set the MPLS MTU less than or equal to the interface MTU to prevent data corruption, dropped packets, and high CPU conditions".

Then , whether I set the mpls mtu > ip mtu or set the mpls mtu < ip mtu?

Now , I meet the device is cisco WS-C6509 with IOS 12.2(18)SXD7b , the interface is a Gigaethernet with the default interface mtu=1500 and mpls mtu=1532, when this port receive a packet labled = 1504 ,the device will drop this ? or fragement it?

I have this problem too.
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shivlu jain Thu, 06/26/2008 - 00:15

Actually on fast ethernet interfaces one cannot configure more than mtu 1500 but when the labeled packet will come it is going to minus the value from the total size menas 4 bytes will be minuesed. Now the packet size is 1496 + 4 bytes=1500 bytes. But on the giga interfaces they support more than 1500 or we can jumbo frames. No fragmentation will be done in that case but it should be clear that the whole LSP path should able to hadle more than 1500 bytes packets; if on the path anyinterface which is hvaing fast ethernet and cannot support more than 1500; thee packet will be dropped there or fragment the packet to deliver it properly.



lijia1980 Thu, 06/26/2008 - 07:29

Thanks for your reply, you mean the giga interface permit the jumbo frames , right?

And I want to know how to set the mpls mtu and interface mtu with the IOS I meet.

guruprasadr Sat, 06/28/2008 - 05:50

HI, [Pls RATE all Informative POST]

Three Kinds of MTU that you can Configure as per your requirement:

IP MTU > ip mtu 1500

MPLS MTU > tag-switching mtu 1546

L2 MTU > mtu 1526

Hope I am Informative

Pls RATE all Informative POST

Best Regards,

Guru Prasad R


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