Cisco 4000 series switches

Tue Jul 16 12:44:40 UTC 2002

 Hello,

> I've pretty much always assumed that what a switch
> reported as a status regarding the link of a node was the
> actual status of the line to be the case. However, when I

I think that there is no such thing as "the actual status of the line".
Each end of the cable has a status, either full or half. That is, they
don't use or do use CSMA/CD.

> Is this a common situation or just a mis-understanding of the column
> that makes up Duplex for the Cisco IOS? Or is it that The Cisco Switch
> truly is trying to do full duplex but the end node is only doing half.

It's quite common, that one end or the cable works in full duplex mode,
while the other works in half duplex. There can be various reasons for
this. Improper configuration (setting on end to auto-negotiate while
setting the other to fix full-10 for example), software/firmware bugs,
etc.

> If so would/should I see line errors at some point under heavy

You should see late collisions on the half duplex end, since the full
duplex end will transmit frames whenever it has them, and this happens too
between the transmission of the 64th octet and the CRC of frames sent out
on the half duplex end, which is a violation of CSMA/CD (late collision).

On the full duplex end, you'll find CRC errors, runts, alignment errors,
because the half duplex end stops transmitting frames in the middle, when
it senses late collisions.

Andras