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New Member

MTU size issue ?

Hi,

My MPLS running both MPLS-VPN and EoMPLS . P-routers connections are TDMs(POS interfaces) , and PE-CEs are ethernet links .I can ping between CEs , but cannot copy shared files neither FTP between sites . I think this would be MTU issue . As now I set all interfaces with MTU 1526 , do you have any recomm about changing it ?

Thanks & Regards,

LUAN

9 REPLIES

Re: MTU size issue ?

Hi,

usually you would take "mpls mtu" command to allow labeled packets larger than the normal interface mtu. Something like:

interface Serial0/0

mpls mtu 1530

In your case the question is, where packets are dropped and which service is involved. Can you localize the problem? There should be interface counters showing you the dropped packets.

MPLS VPN traffic needs 2 labels, i.e. 8 Byte in addition. EoMPLS needs at least 1518 + 8 = 1526 Bytes, possibly 4 Byte more for a control word, with MPLS TE, FRR, etc. there could be additional labels, i.e. a larger MTU required.

Also make sure all your label switch interfaces do support the larger MTU.

Regards, Martin

Re: MTU size issue ?

If you have any LAN switches between your MPLS-enabled routers (P-P and P-PE), you have to ensure that all involved links on all involved switches are configured for MTU corresponding to that of your routers.

Beware that in some newer IOS'es command 'mpls mtu' will no longer accept values greater than interface MTU unless you specify keyword 'override'. For these new releases Cisco recommends to first change interface MTU. After this you no longer need to change 'mpls mtu', however you may need to change 'ip mtu'.

New Member

Re: MTU size issue ?

Hi,

Thanks for your replies .

All corresponding interfaces are set mpls mtu 1526 but still cannot transfer files . But when I set " ip tcp adjustt-mss 1420 " on the PE-to-CE vrf interface then it works well .

LUAN

Re: MTU size issue ?

Check LAN switches that interconnect your routers - they also must have MTU set to at least 1526, else packets will be sent from routers but will be dropped by switches.

New Member

Re: MTU size issue ?

Change your system mtu to 1546 on routers included ISP router on the other end.

Routers reboot to take affect. Then show system mtu in router to see if it update change.

Show system mtu 1546

New Member

Re: MTU size issue ?

Have you tried poinging with DF bit off, and see how big packets you can ping with before they are fragmented?

New Member

Re: MTU size issue ?

I have a network of Cisco 7200 routers some G1 and some NPE400, I can't seem to change the MTU on the fast ethernet interfaces on the NPE-400 routers i get this prompt:

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

All routers are connected using a switch network on a dot1q vlan subinterface.

I've had to use the ip tcp asjust-mss 1456 to get tcp connections working over the network, however this does not work for assymetrical connections that a customer may go via another provider and back through us the session hangs unless they adjust their outbound tcp mss value lower.

All the routers are connected via Cisco 2970 and Cisco 3750 switches with their system mtu increased to their maximums, so I can change the mtu on the switch interfaces but if I can't on the 7200 NPE 400 not sure what I should do to fix these assymetrical issues?

New Member

Re: MTU size issue ?

try setting mtu size 1546 on a router, not interface. It will cover interface for you.

system mtu 1546

New Member

Re: MTU size issue ?

Hi all,

Thank you all for your clarifications .

I got the bug and it was on 1 P-P connection; there was 1 ethernet interface of a carrier (this I forgot to describe in 1st msg , sorry) that I was not able to interfere to change it's MTU(1500) . So I decided to disable MPLS on this link , then it worked .

Conclusion : check & change to suitable MTU size, point by point , on all links related throughout the Provider network.

Thanks & Regards,

Luan

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