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Is it MPLS mtu problem ?

Our compny is serviceing Mpls between HQ and Branch. PE<->PE is running EBGP through ppp multilink and PE<->CE is running OSPF.The PE router is a cisco 3845.When I configure, it is working well. In case CE<->CE, I can a ping each other, but I can not access web-server from Branch 2day later. I thought it is MTU problem,So that is way I have changed PE<->PE mtu size as 1526 and CE<->CE 1436. It was working well web-server,FTP at time. What am I asking you, I can access some kind of file.

Do I have to re-adjust Mtu size ? If do I have to do, which position do I have to do it ? Let me know. I am in serious.


Re: Is it MPLS mtu problem ?


You might have a MTU issue, but maybe not in the MPLS environment.

Usually hosts are connected to the network through ethernet and have a MTU of 1500 Bytes. Thus your CE setting of 1436 Bytes might create your observed problem. The CE router might have to discard packets above 1436 with DF bit set.

MPLS adds 2 labels, i.e. 8 Bytes. With your settings this should create no problem (1436+8=1444 < 1526).

The general recommendadtion thus is: CE <-> CE should be 1500 Byte and inside the MPLS network you should adjust MTU to allow this.



interface Serial0

description to PE Serial1/0

mtu 1500


interface Serial1/0

description to CE Serial0

mtu 1500

interface Serial1/1

description to MPLS cloud

mtu 1526

mpls ip

To localize your problem I would use an extended ping with df bit set and varying sizes. You should get 1436 Bytes with your current settings. Increase this to 1500 Bytes MTU and test again. Finally you should be able to rule out network related issues - there might still be application related issues like file access restrictions on the server.

Hope this helps! Please use the rating system.

Regards, Martin

New Member

Re: Is it MPLS mtu problem ?

Thank you, Martin

I am going to try it what you recommand.

I hope it work well.

Let me know,if you have any good idea more.

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