RB

If you configure portfast then STP does still run in that port. Portfast does not disable STP but only changes the behavior a little so that instead of going through the stages of listening and learning before going to forwarding with portfast the port goes immediately into the forwarding state. But it does still run STP.

If you connect a switch on that port it has the possibility that you could connect another port on the second switch to a port on the first switch and create a loop. If the loop is created and portfast is enabled there will be a great increase in traffic over those ports until STP is able to detect the loop and force one of the ports into blocking mode.

HTH

Rick