One of my sysadmins created a loop on one of my 2948s. He daisy chained the ethernet console ports of a bunch of suns and patched the top and bottom machine to the 2948. The ports on the 2948 were all configured to portfast enabled. Yet somehow the switch still figured out there was a loop and put one of the ports into spanning tree blocked state. How did the switch succeed in blocking one port depsite there being portfast enabled? Thanks.
The original post seems to have assumed that postfast would disable spanning tree (which is a very common - but incorrect assumption). The two responses were quite correct in explaining that portfast does not disable spanning tree but only puts the port immediately into forwarding mode. The port continues to process BPDUs and to participate in the spanning tree. Since BPDUs are sent every two seconds there will be a check for possible loop every two seconds.