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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 16 Configuring Optional Spanning-Tree Features
Configuring Optional Spanning-Tree Features
Beginning in privileged EXEC mode, follow these steps to enable Port Fast. This procedure is optional.
Note You can use the spanning-tree portfast default global configuration command to globally enable the
Port Fast feature on all nontrunking NNIs.
To disable the Port Fast feature, use the spanning-tree portfast disable interface configuration
command.
Enabling BPDU Guard
When you globally enable BPDU guard on NNIs that are Port Fast-enabled (the interfaces are in a Port
Fast-operational state), spanning tree shuts down Port Fast-enabled NNIs that receive BPDUs.
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. The BPDU guard
feature provides a secure response to invalid configurations because you must manually put the interface
back in service. Use the BPDU guard feature in a service-provider network to prevent an access port
from participating in the spanning tree.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
interface interface-id Specify an interface to configure, and enter interface configuration
mode. If the interface is not an NNI, you must enter the port-type nni
interface configuration command before enabling Port Fast.
Step 3
spanning-tree portfast [trunk] Enable Port Fast on an access port connected to a single workstation or
server. By specifying the trunk keyword, you can enable Port Fast on a
trunk port.
Note To enable Port Fast on trunk ports, you must use the
spanning-tree portfast trunk interface configuration command.
The spanning-tree portfast command does not work on trunk
ports.
Caution Make sure that there are no loops in the network between the
trunk port and the workstation or server before you enable
Port Fast on a trunk port.
By default, Port Fast is disabled on all NNIs. UNIs do not participate in
STP.
Step 4
end Return to privileged EXEC mode.
Step 5
show spanning-tree interface
interface-id portfast
Verify your entries.
Step 6
copy running-config startup-config (Optional) Save your entries in the configuration file.