How to view mpls mtu problem

Unanswered Question
Apr 22nd, 2007


I would like to know how to view on Cisco routers if there are drop packet occuring over the mpls network with mtu sizes issue.

I'm implementing mtu changes on the router's physical interface instead of using the command "mpls mtu" and another one, the DWDM link being used by the mpls routers are set to 2000 bytes mtu. Could this setup cause to drop packets?



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mheusinger Tue, 04/24/2007 - 08:22

Hi Mel,

The first question, which comes to my mind is: which other devices are involved besides routers and what is their MTU setting?

It could be the router can forward the labeled packets, but the other devices (DWDM, etc.) can not.

Whether 2000 Bytes can cause the problem depends on your access line MTU into the MPLS cloud. If the access MTU is 1500 (default) then labeled packets will not be larger than 2000 Byte and no problems arise.

To check for MTU related problems you can use an extended ping and use the sweep option with DF bit set. An example with packet sizes from 1490 to 1501 Bytes (without labels) is given below:


Protocol [ip]:

Target IP address:

Repeat count [5]: 1

Datagram size [100]:

Timeout in seconds [2]:

Extended commands [n]: yes

Source address or interface:

Type of service [0]:

Set DF bit in IP header? [no]: yes

Validate reply data? [no]:

Data pattern [0xABCD]:

Loose, Strict, Record, Timestamp, Verbose[none]:

Sweep range of sizes [n]: yes

Sweep min size [36]: 1490

Sweep max size [18024]: 1501

Sweep interval [1]:

Type escape sequence to abort.

Sending 12, [1490..1501]-byte ICMP Echos to, timeout is 2 seconds:

Packet sent with the DF bit set


Success rate is 58 percent (7/12), round-trip min/avg/max = 1140/1142/1148 ms

The result tells that up to 1492 Bytes was ok, but above packets were dropped. In this setup there was one label in use and no MTU adjustment done, thus the "missing" 4 Bytes.

The "M" tells you an ICMP message was received "Fragmentation needed, but df bit set".

Hope this helps!

Regards, Martin

dtushing123 Wed, 04/25/2007 - 05:33

Martin, this is a very useful tool and I'm going to put this post in my archive.

How can you tell that the packet size up to 1492 was okay from the output ?

mheusinger Wed, 04/25/2007 - 23:22

Thank you Swaroop, 1496 is the correct number.

Each "!" represents a successful ICMP message and each message has 1 Byte more than the previous one. 7 times "!" means 1490, 1491, ..., 1496 was ok.

Regards, Martin

mel_bs Thu, 04/26/2007 - 00:16


Thanks for your prompt reply...

The setup is like

PE1 <-> DWDM <-> P1 <-> DWDM <-> PE2

P/PE router interfaces are set to "mtu 1550" while the DWDM are set to 2000 bytes mtu.


1. Is it possible that packets are drop at the ingress interface of the routers?

2. Does the "show interface" command on the router displays dropped packets?



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