mtu on ethernet interface

Unanswered Question
May 29th, 2008

Hi all, I thought that the maximum mtu for ethernet was 1500, I tried to ping my router with 1500 bytes of data and it did not work, any reason for this ?

I have this problem too.
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solpandor Thu, 05/29/2008 - 06:01

Carl,

when you say it doesnt work, can you be more specific, do you get request timed out, host unreachable? It could be that ICMP is disabled on your interface

Sol

guruprasadr Thu, 05/29/2008 - 06:13

HI Carl, [Pls Rate if HELPS]

Max MTU supported on Ethernet Interface is 1500 size.

In this case, you should be able to PING upto to size of 1496 only. The rest 4" bytes are the VLAN Header added in the Path.

So, the situation will be, 1496 + 4" bytes VLAN Header.

You need Jumbo Frame Support to PING upto 1500 bytes.

Refer Link below:

http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml

Hope I am Informative.

Pls Rate if HELPS

Best Regards,

Guru Prasad R

michael.leblanc Thu, 05/29/2008 - 10:26

You're forgetting the IP header (20 bytes).

Pinging with 1500 bytes of "data" will require fragmentation. The two packets sent will have a common identifier in the IP header to aid re-assembly on the receiving end.

The first packet will have the "more fragments bit" set in the IP header. The second packet will not.

The first packet will identify ICMP as the next layer protocol, but will not contain an ICMP header. This packet will accommodate 1480 packets of data.

The second packet will identify ICMP as the next layer protocol; contain an ICMP header (8 bytes I think); and the remaining 20 bytes of data.

Your ping may fail due to the requirement for fragmentation, depending on configuration of your infrastructure.

e.g.:

Some of our router interfaces are configured to drop fragments.

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