20-2
Cisco IE 3000 Switch Software Configuration Guide
OL-13018-03
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 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
configuration, such as the connection of an unauthorized device, and the BPDU guard feature puts the
port in the error-disabled state. When this happens, the switch shuts down the entire port on which the
violation occurred.
To prevent the port from shutting down, you can use the
errdisable detect cause bpduguard shutdown
vlan
global
configuration
command to shut down just the offending VLAN on the port where the
violation occurred.
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