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Configuring PortFast
PortFast should be used only when connecting a single end station to a switch port. If you enable PortFast
Caution
on a port connected to another networking device, such as a switch, you can create network loops.
When the switch powers up, or when a device is connected to a port, the port normally enters the
spanning tree listening state. When the forward delay timer expires, the port enters the learning state.
When the forward delay timer expires a second time, the port is transitioned to the forwarding or
blocking state.
When you enable PortFast on a port, the port is immediately and permanently transitioned to the
spanning tree forwarding state.
Configuring PortFast
These sections describe how to configure spanning tree PortFast on the switch:
•
•
Enabling Spanning Tree PortFast
Caution
PortFast should be used only when connecting a single end station to a switch port. If you enable PortFast
on a port connected to another networking device, such as a switch, you can create network loops.
To enable PortFast on a switch port, perform this task in privileged mode:
Task
Step 1
Enable PortFast on a switch port connected to a
single workstation or server.
Step 2
Verify the PortFast setting.
This example shows how to enable PortFast on port 1 of module 4 and verify the configuration (the
PortFast status is shown in the "Fast-Start" column):
Console> (enable) set spantree portfast 4/1 enable
Warning: Spantree port fast start should only be enabled on ports connected
to a single host.
a fast start port can cause temporary spanning tree loops.
Spantree port 4/1 fast start enabled.
Console> (enable) show spantree 4/1
Port
--------- ----
4/1
4/1
4/1
4/1
4/1
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
8-2
Chapter 8
Enabling Spanning Tree PortFast, page 8-2
Disabling Spanning Tree PortFast, page 8-3
Connecting hubs, concentrators, switches, bridges, etc. to
Vlan
Port-State
-------------
1
blocking
100
forwarding
521
blocking
522
blocking
523
blocking
Configuring Spanning Tree PortFast, UplinkFast, and BackboneFast, and Loop Guard
Command
set spantree portfast mod_num/port_num enable
show spantree mod_num/port_num
Cost
Priority
Fast-Start
-----
--------
----------
19
20
enabled
10
20
enabled
19
20
enabled
19
20
enabled
19
20
enabled
Use with caution.
Group-method
------------
78-12647-02
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Troubleshooting
