Sims,
What does it mean of “disables BPDU filtering and acts as a normal interface ” ?
It means that the switch realizes either there has been a change in topology, or the administrator has made an error. A BPDU should never be received on an interface on which BPDU filtering is enabled. When the filtering is enabled globally, this is a safety mechanism so that when a BPDU is received on a port where the global filtering was enabled, the Switch knows there must be another switch on the other side. In order to prevent a possible loop, the BPDU filtering is turned off just for this port, the portfast feature is disabled, and the switch will have this port go through the full spanning-tree states (instead of skipping straight to Forwarding).
What if there is portfast enabled on these interface and enable bpdufilter also ? If this disable spanning tree , There is no use of portfast?
Note that the method of enabling bpdu filtering locally at a port level does not have the same safety mechanism as globally enabling it (as was discussed above). Without the safety mechanism, there is a much higher chance that a loop can be created, and for this reason, most people try to avoid setting bpdu filtering at a port level.
I suspect that even with BPDU Filtering enabled for a port without having PortFast enabled, the port will still go through all STP states (Listening, Learning, Forwarding for regular STP). In other words, even if the switch would never receive or send a BPDU where filtering is disabled, it would still “go through the motions” of normal STP without PortFast telling it to skip ahead. If this is true, PortFast would still have a purpose.
I would encourage you to test this yourself and see what happens–I would like to know!