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18-2
Catalyst 2960 and 2960-S Switch Software Configuration Guide
OL-8603-09
Chapter 18 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
• Understanding Root Guard, page 18-10
• Understanding Loop Guard, page 18-11
Understanding Port Fast
Port Fast immediately brings an interface configured as an access or trunk port to the forwarding state
from a blocking state, bypassing the listening and learning states. You can use Port Fast on interfaces
connected to a single workstation or server, as shown in Figure 18-1, to allow those devices to
immediately connect to the network, rather than waiting for the spanning tree to converge.
Interfaces connected to a single workstation or server should not receive bridge protocol data units
(BPDUs). An interface 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 interfaces must wait for spanning-tree to
converge, it is effective only when used on interfaces connected to end stations. If you enable Port Fast
on an interface connecting 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 18-1 Port Fast-Enabled Interfaces
Understanding BPDU Guard
The BPDU guard feature can be globally enabled on the switch or can be enabled per port, but the feature
operates with some differences.
At the global level, you 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 them. In a valid configuration, Port Fast-enabled
ports do not receive BPDUs. Receiving a BPDU on a Port Fast-enabled port means an invalid
WorkstationsWorkstations
Server
Port
Fast-enabled port
Port
Fast-enabled
ports
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