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?
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:
Target IP address: 10.1.1.1
Repeat count : 1
Datagram size :
Timeout in seconds :
Extended commands [n]: yes
Source address or interface:
Type of service :
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 : 1490
Sweep max size : 1501
Sweep interval :
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
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".
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