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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 21 Configuring Optional STP Features About PortFast
To disable the EtherChannel guard feature, use the no spanning-tree etherchannel guard misconfig
global configuration command.
Use the show interfaces status err-disabled privileged EXEC command to show which switch ports are
disabled because of an EtherChannel misconfiguration. On the remote device, you can enter the show
etherchannel summary privileged EXEC command to verify the EtherChannel configuration.
After the configuration is corrected, enter the shutdown and no shutdown interface configuration
commands on the port-channel interfaces that were misconfigured.
About PortFast
Spanning Tree PortFast causes an interface configured as a Layer 2 access port to enter the forwarding
state immediately, bypassing the listening and learning states. Use PortFast on Layer 2 access ports
connected to a single workstation or server to allow those devices to connect to the network immediately,
rather than waiting for spanning tree to converge. Even if the interface receives a bridge protocol data
unit (BPDU), spanning tree does not place the port into the blocking state. Spanning tree sets the port’s
operating state to non-port fast even if the configured state remains port fast and starts participating in
the topology change.
Note Because the purpose of PortFast is to minimize the time that access ports must wait for spanning tree to
converge, it is most effective when used on access ports. If you enable PortFast on a port connecting to
another switch, you risk creating a spanning tree loop.
Enabling PortFast
Caution Use PortFast only when connecting a single end station to a Layer 2 access port. Otherwise, you might
create a network loop.
To enable PortFast on a Layer 2 access port to force it to enter the forwarding state immediately, perform
this task:
Step 4 Switch(config)# show spanning-tree summary Verifies your entries.
Step 5 Switch(config)# copy running-config
startup-config (Optional) Saves your entries in the configuration file.
Command Purpose
Command Purpose
Step 1 Switch(config)# interface {{fastethernet |
gigabitethernet | tengigabitethernet} slot/port}
| {port-channel port_channel_number}
Specifies an interface to configure.
Step 2 Switch(config-if)# [no] spanning-tree portfast Enables PortFast on a Layer 2 access port connected to a
single workstation or server.
Use the no keyword to disable PortFast.