PortFast permanently places a switch port in the Spanning Tree Protocol (STP) forwarding state, bypassing the listening and learning states. PortFast is a feature available on Catalyst switches that enables faster connectivity for hosts connected to an accesslayer switch port. If PortFast is not enabled, a switch port transitions through the STP listening and learning states before it enters the forwarding state. This process can take as long as 30 seconds if the default STP timers are used.

Additionally, port initialization could take as long as 50 seconds if Port Aggregation Protocol (PAgP) is enabled. Since host computers or IP phones do not typically perform bridging functions, it is not necessary to make the switch port transition through the normal STP states, because the port should not encounter a switching loop. Thus STP skips the listening and learning states and places the port into the forwarding state so that the end host has immediate network connectivity.

Although PortFast does accelerate the STP process, PortFast does not disable STP on the port.

Host ports that are not enabled for PortFast can cause a high number of STP topologychanges to flood throughout the network, thereby causing high CPU utilization on network switches. Therefore, you should enable PortFast on ports that are connected to end hosts, such as IP phones, client workstations, or servers. Typically, servers and client workstations do not perform routing or switching, so there is no need to delay network connectivity while STP cycles through the listening and learning states. Conversely, PortFast should not be enabled on a port that is connected to a switch or other networking device. If you enable PortFast on such a port, you risk creating switching loops because the port is permanently in the STP forwarding state.

PortFast can be enabled as a global default as well as on a specific port. If you enable PortFast as a global default, each port that is configured as an access port is enabled with PortFast. You can enable PortFast as a global default by issuing the spanningtree portfast default global configuration command. You can also enable PortFast on a perport basis by issuing the spanningtree portfast interface configuration command.