04-22-2007 07:41 PM
Hi,
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?
Thanks,
Mel
04-24-2007 08:22 AM
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:
R#ping
Protocol [ip]:
Target IP address: 10.1.1.1
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 10.1.1.1, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!!!M.M..
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
04-25-2007 05:33 AM
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 ?
04-25-2007 05:59 AM
There must have been a typo , Martin must have meant 1496.
Here is a link explaining the extended ping and trace command.
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f22.shtml
HTH-Cheers,
Swaroop
04-25-2007 11:22 PM
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
04-26-2007 12:16 AM
Martin,
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.
Questions:
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?
Mel
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