MPLS MTU on FE interfaces with fixed interface MTU

Unanswered Question
Dec 24th, 2007

What is the recommendation for setting MPLS MTU when the hardware does not permit the setting of the interface MTU:

PE-Edge-2(config)#int fastEthernet 0/0

PE-Edge-2(config-if)#mtu 1524

% Interface FastEthernet0/0 does not support user settable mtu.

The MPLS MTU could be changed with "mpls mtu 1524", but this gives the error:

PE-Edge-2(config-if)#mpls mtu 1524

*Dec 24 20:54:43.467: %TFIB-3-MPLS_MTU_SET: Setting mpls mtu to 1524 on FastEthernet0/0 which is higher than the interface mtu 1500 . This could lead to packet forwarding problems including packet drops.

Is it permissible to set the MPLS MTU greater than the interface MTU, as I thought that MPLS packets cannot not be fragmented, but rather simply dropped?

In this scenario, is the only solution to limit the size of inbound packets with something like "ip tcp adjust-mss 1476" to avoid any potential packet drops?

I have this problem too.
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ashish_gupta Mon, 12/24/2007 - 13:40


In this case both mpls mtu and ip mtu should be 1500. your MPLS mtu in most of the cases will be 4 bytes more. hence 1524 doesnt seems to be a configure settings.


Aasheesh Gupta

mmelbourne Wed, 01/02/2008 - 02:21

On switches, I can configure jumbo frame support, increase the MPLS MTU and everything should be OK.

What is best practice when an router's physical interface MTU cannot be changed from 1500? Is it appropriate to specify an MPLS MTU greater than 1500?

cpubob Thu, 01/03/2008 - 13:01

if your doing AToM then you have to change the mpls mtu in order to transport full frames over your backbone. I have the same issue with the mtu and setting the mpls mtu to 9196 while leaving the mtu at 1500 did not impact regular traffic at all but it did allow our l2vpn's to function correctly.


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