You can tune the spanning-tree timers to adjust the timing, but these timers are
meant to be set to the default value. The default values are put in place to give the network
enough time to gather all the correct information about the network topology.
Spanning-tree PortFast causes an interface that is con?¬?gured as a Layer 2 access port to
transition immediately from the blocking state to the forwarding state, bypassing the
listening and learning states. You can use PortFast on Layer 2 access ports that are
connected to a single workstation or server to allow those devices to connect to the network
immediately rather than wait for spanning tree to converge. Figure 2-25 shows access ports
connected with PortFast enabled.
Figure 2-25 PortFast
If an interface that is con?¬?gured with PortFast receives a BPDU, then spanning tree can
transition the port to the blocking state. Using a feature called BPDU guard, the port can be
disabled completely when it receives a BPDU to prevent any potential loops caused by
PortFast.
CAUTION Because the purpose of PortFast is to minimize the time that access ports
must wait for spanning tree to converge, you should use it only on access ports.
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