Cisco 3750G - Catalyst Integrated Wireless LAN Controller, Руководство по настройке

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20-2
Catalyst 3750 Switch Software Configuration Guide
OL-8550-02
Chapter 20 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 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 20-1 , 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 20-1 Port Fast-Enabled Interfaces
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 enable BPDU guard on Port Fast-enabled interfaces by using the spanning-tree
portfast bpduguard default global configuration command. Spanning tree shuts down interfaces that
are in a Port Fast-operational state if any BPDU is received on those interfaces. In a valid configuration,
Port Fast-enabled interfaces do not receive BPDUs. Receiving a BPDU on a Port Fast-enabled interface
signals an invalid configuration, such as the connection of an unauthorized device, and the BPDU guard
feature puts the interface in the error-disabled state.
Workstations Workstations
Server
Port
Fast-enabled port
Port
Fast-enabled
ports
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