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15-2
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 15 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Understanding Port Fast
Port Fast immediately brings an interface configured as an access or trunk port from a blocking state to
the forwarding state, bypassing the listening and learning states. You can use Port Fast on ports
connected to a single workstation or server, as shown in
Figure 15-1, to allow those devices to
immediately connect to the network, rather than waiting for the spanning tree to converge.
Ports connected to a single workstation or server should not receive bridge protocol data units (BPDUs).
A port with Port Fast enabled goes through the normal cycle of spanning-tree status changes when the
switch is restarted.
Note Because the purpose of Port Fast is to minimize the time ports must wait for spanning-tree to converge,
it is effective only when used on ports connected to end stations. If you enable Port Fast on a port
connected to another switch, you risk creating a spanning-tree loop.
You can enable this feature by using the spanning-tree portfast interface configuration or the
spanning-tree portfast default global configuration command.
Figure 15-1 Port Fast-Enabled Ports
Understanding BPDU Guard
The BPDU guard feature can be globally enabled on the switch or can be enabled per interface, but the
feature operates with some differences.
At the global level, you can enable BPDU guard on Port Fast-enabled ports by using the spanning-tree
portfast bpduguard default global configuration command. Spanning tree shuts down ports that are in
a Port Fast-operational state if any BPDU is received on those interfaces. In a valid configuration, Port
Fast-enabled ports do not receive BPDUs. Receiving a BPDU on a Port Fast-enabled port signals an
invalid configuration, such as the connection of an unauthorized device, and the BPDU guard feature
puts the port in the error-disabled state.
At the interface level, you can enable BPDU guard on any port by using the spanning-tree bpduguard
enable interface configuration command without also enabling the Port Fast feature. When the port
receives a BPDU, it is put in the error-disabled state.
WorkstationsWorkstations
Server
Port
Fast-enabled port
Port
Fast-enabled
ports
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