Forum Discussion
Hamish
Jan 21, 2012Cirrocumulus
The 'problematic' configurations when specifying speed and duplex on a cisco and auto on the connecting interface is due to the fact that cisco read the specs slightly differently from others... When you specify duplex on a cisco switch port then the switch no longer advertises the duplex to the connected port. Speed is easy (That's voltage). But duplex needs advertising. However cisco reads the spec as saying that if duplex is hard-set, then you don't advertise any more.
The sad part of that is that if you hard-set the cisco switch port to full-duplex and have auto/auto on the connected port, then speed is detected by voltage, but because there's no advertising the connected switch port chooses half-duplex (because a half-duplex hub doesn't advertise).
A connection that's full-duplex at one end and full at the other then generates unexpected collisions... It'll work fine at low speeds, but if you try to push too much data through it, it will just crawl. Also some versions of cisco (Catos especially, but you can configure IOS to do the same) will disable the switch port if it's getting errors (And collisions on a full-duplex port is an error).
Most other systems advertise when you hard-set duplex... e.g. Nokia, AIX, Solaris... It's safe to say that unless you set a cisco to auto duplex, you'll probably get problems (UNless you're willing to put up with the pain of hard-setting ALL your devices. Not sure why you would, but I have seen it done).
H