4506 Problem, not passing full frames.

[nialltodd]

Hi all. I have had a problem over the last couple of days with my pair of 4506 switches. The problem started when users noticed they could ping to attached servers but not use them as per required. I discovered that the 4506 was passing frames as desired when pinged (from an NT desktop) with a 1446 byte ping, but 1447+ was conastantly dropped. When pinging from a router, 1476 byte packets were fine, but 1477+ dropped. This did explain why services were not available to/from the servers (default MTU being larger than the above). I was not able to pinpoint the source of this problem, but a reload of both switches cured it.

Is anybody else aware of such a problem? This is the latest in a series of problems we have had with the 4506, but I am unable to find any official info relating to it.

Both switches are running Cat4000-I5S-M, 12.1(20)EW1.

Any advice or experience passed on would be much appreciated.

Thanks, Niall.

[peter.mainwaring]

It sounds a problem with fragmentation, but I can't think of a reason why it would suddenly start happening.

You can test the use of the "don't fragment" bit by running a ping with the "don't fragment" bit set and the buffer size set. Here is an example:-

C:\>ping -f 10.96.7.1 -l 1472

Pinging 10.96.7.1 with 1472 bytes of data:

Reply from 10.96.7.1: bytes=1472 time<10ms TTL=255

Reply from 10.96.7.1: bytes=1472 time<10ms TTL=255

Reply from 10.96.7.1: bytes=1472 time<10ms TTL=255

Reply from 10.96.7.1: bytes=1472 time<10ms TTL=255

C:\>ping -f 10.96.7.1 -l 1473

Pinging 10.96.7.1 with 1473 bytes of data:

Packet needs to be fragmented but DF set.

Packet needs to be fragmented but DF set.

Packet needs to be fragmented but DF set.

Packet needs to be fragmented but DF set.

1472 is the maximum size of the packet when you take into account the headers (20 bytes for the IP header and 8 for the ICMP header). This size refers to ICMP "padding" only.

The default MTU that you mention probably doesn't take the header lengths into account.

Not sure if this is going to be much help, but it might go some way to explaining the reason why the 1446 or 1476 pings are ok, but 1447+ or 1477+ aren't.

Pete