I have read at least 6 links on Cisco Systems to try and really understand the difference between UDLD Enable and UDLD Aggressive. The more I read the more I am confused. My final understanding is as follows and I wish someone to correct me if my understanding is correct.
UDLD Enable just detects if a link becomes Unidirectional - that is - it can send UDLD frames but cannot receive UDLD frames, but takes no action when this happens.
UDLD Aggressive on the other hand shuts down the interface when it can send UDLD frames but cannot receive them and hence called aggressive because this shuts down the interface.
But then I go and read another URL and I find that UDLD Enable also will shut down the interface when it can transmit UDLD frames but cannnot receive UDLD frames from teh other end.
It would be hard to simulate this in the lab because if we unplug one fibre cable from switch A to switch B the interface state goes to LINE DOWN / PROTOCOL DOWN on both switches and we cannot see UDLD playing any role at all - be it UDLD Enable or UDLD Aggressive.
To simulate it, we would have to some how keep the Interface in LINE UP / PROTOCOL UP state on both switches but somehow stop UDLD frames from switch A being received by switch B.
Or is there any other easy way of simulating this in the lab to see the difference between UDLD Enable and UDLD Aggressive behaviours.