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New Member

MTU related issues with FRoMPLS

Hi Folks,

While using FRoMPLS I am experiencing limitations in the Edge MTU. The

edge MTU is limited to 1492 bytes above which the packets are droped. The MPLS MTU between PE routers has been changed to 1526 bytes. I understand AToM does not do fragmentation. I did see a workaround in he Web saying increasing the Core MTU to carry the Edge MTU of 1500. This does not seem to work. Any change in the core MTU does not reflect in the edge MTU. Can anyone help me isolae the issue.

thanks in advance



Re: MTU related issues with FRoMPLS

I am just wondering if there is any command that can be configured on the edge to do a path MTU discovery which will enable the change in core MTU to be reflected in the edge.

New Member

Re: MTU related issues with FRoMPLS

Hello Netpros,

I have got the answers from Netpro as well as from Cisco TAC. Here is goes..

AToM as you all know does not support fragmentation of packets. The overhead added to the packet initiated from the end PC connecting to the CPE, has to be compromised by increasing the core MTU. However this change cannot be done in a Fastethernet / Ethernet Interfaces.

The solutions could be,

1. Avoid Fastethernet / Ethernet interfaces in the LSP between PEs.


2. Use only Gigabit Ethernets if at all a required in the LSPs, as it supports user settable MTUs.


3. Change the edge MTU to 1400 bytes(MTU of the PC NIC) as suggested by Microsoft. refer url below.

New Member

Re: MTU related issues with FRoMPLS

the way i usually solves these kind of issues of mtu is:

if you suspect the problem is somwhere in your core between both pe routers

change the atom mpls implementation just for the diagnostic to be a layer3 vpn then from the ce do an extended ping where you set the df bit to 1 and the size to 1500 so routers in the core will not fragment the packet then do "debug ip icmp" on the ce and then ping the other ce

the result of this action is you will see with the debug the hop that cannot do fragmentation .(icmp cannot fragment)

and on these routers in your core you should concentrate and change the mtu with the t"ag-switching mtu 1526" command.

from my experience sometimes usually on ethernet interfaces (not giga)and regular serial if you enlarge the mtu it is not inuff you should also shut and unshut the interface and sometimes even reload the i hope large part of your core is pos and giga.

also because atom does not support fragnentation make sure each link connecting the ce to pe on both sides have the same mtu

after you see that with a layer 3 vpn there is no fragmentation then switch back to atom.

good luck


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