PortFast

PortFast
By default, the Spanning Tree Protocol (STP) runs on all ports on a switch. Because most of the
ports connect to workstations, printers, servers, routers, and so on, it’s basically a waste of
resources for these point-to-point ports to be running the Spanning Tree Protocol. When a
device—let’s say, a workstation—powers up, it takes up to 50 seconds before the switch forwards
data on the port, because the STP is making sure no loops are going to occur when the
port is in forwarding mode. Not only is this a waste of time (because a loop does not occur with
point-to-point links), but some protocols or applications could time out.
PortFast is used to make a point-to-point port almost immediately enter into forwarding state
by decreasing the time of the listening and learning states. This is very helpful for switch ports that
have workstations or servers attached, because these devices will connect immediately instead of
waiting for the STP to converge. If you connect a hub to a port configured with PortFast and then
accidentally connect another port into the switch from the hub, you will have a network loop, and
STP will not stop it. It is important to make sure that PortFast is used only on point-to-point links
connected only to workstations or servers.