Port fast disables STP on a port allowing the interface to start forwarding immediately by bypassing STP safety checks.
This should be used with caution and only on ports NOT connected to other switches!
2.7.b BPDU guard
BPDU guard will immediately place the port in an err-disabled state if it detects and STP BPDUs on that interface.
Enable BPDU guard if enabling PortFast.
IOU1(config-if)#spanning-tree bpduguard enable IOU1(config-if)# *May 6 13:59:00.436: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to down IOU1(config-if)# *May 6 13:59:01.436: %LINK-3-UPDOWN: Interface Ethernet0/0, changed state to down IOU1(config-if)#^Z IOU1#show inf *May 6 14:00:13.974: %SYS-5-CONFIG_I: Configured from console by console IOU1#show int e0/0 Ethernet0/0 is down, line protocol is down (err-disabled)