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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
78-17058-01
Chapter 16 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Understanding Port Fast
Port Fast immediately brings an NNI 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 NNIs connected
to a single workstation or server, as shown in Figure 16-1, to allow those devices to immediately connect
to the network, rather than waiting for the spanning tree to converge.
NNIs connected to a single workstation or server should not receive bridge protocol data units (BPDUs).
An NNI 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 NNIs 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 16-1 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 NNIs by using the spanning-tree
portfast bpduguard default global configuration command. Spanning tree shuts down NNIs that are in
a Port Fast-operational state if any BPDU is received on those NNIs. In a valid configuration, Port
Fast-enabled NNIs do not receive BPDUs. Receiving a BPDU on a Port Fast-enabled NNI 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.
At the interface level, you enable BPDU guard on any NNI by using the spanning-tree bpduguard
enable interface configuration command without also enabling the Port Fast feature. When the NNI
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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