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MTU Issue - Unable to Ping for configured size

Hi All,

 

Recently there was change done on Wan link where the MTU was set to 9000. The same value has been set on the ISP end too. The Network connectivity is as given: CE1 ---- PE1 ---- PE2 ---- CE2

On the CE Interfaces connecting to PE's, the MTU has been set to 9000. ISP is informing that their end is also set to 9000. But the Max size i am able to Ping and get a reply is for 8982 and not 9000, that is, with or without the DF bit being set.

Ethernet is the media used. Router is ASR1006 and the Interface is on Module SPA-10X1GE-V2

 

10.1.1.2 is the CE2 Interface IP addr. NOTE. Hostname and IP addresses are modified to conceal the original.

 

CE1#ping 10.1.1.2 size 8982

Type escape sequence to abort.
Sending 5, 8982-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 5/5/6 ms

CE1#ping 10.1.1.2 size 8983

Type escape sequence to abort.
Sending 5, 8983-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

 

 

CE1#ping 10.1.1.2 size 8982 df-bit

Type escape sequence to abort.
Sending 5, 8982-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 5/9/24 ms

CE1#ping 10.1.1.2 size 8983 df-bit

Type escape sequence to abort.
Sending 5, 8983-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:
Packet sent with the DF bit set
.....
Success rate is 0 percent (0/5)

 

Is this an expected behavior or something more should be done to make this work?

 

Everyone's tags (6)
2 REPLIES 2
Participant

Hi, Praveen Selvam.Don't

Hi, .

Don't forget that MTU at interface is for L2 frames. ICMP traffic has a L3 header that takes some bytes from L2 Data frame (in fact from MTU). You can also look at this discussion about ICMP: https://supportforums.cisco.com/discussion/11353141/icmp-packet-size

Best Regards.

Participant

Hi Praveen,I think it is ok,

Hi Praveen,

I think it is ok, if you add the L2 ethernet overhead (18 bytes) you get the 9000 bytes.

Hope this helps,

Jose.

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