Yesterday we had a strange outage. 45 Access layer switches (2950C's) all lost connectivity to the network. Not all switches terminate on the same distribution layer device (6500).
We had to visit every switch and discovered that the uplink port (fa0/25) had error diabled due to the following:
I have only recently enable aggressive mode UDLD and is the only commonality between the switches affected, i.e they all connect back to a native IOS 6500 with A-UDLD enabled globally.
All the access switches still had normal mode UDLD enabled. I was planning on changing them to aggressive at a later date.
I know that the loopback error message is supposed to be caused by a loop in the keepalive packet. Is there any chance that this condition could have been caused by the two different modes of UDLD interacting ?
I cannot see any other reason as to why 45 switches, in different locations would all lose connectivity to the network at the same time.
Any help is appreciated.