Cisco 2950G 24 - Catalyst Switch Software Configuration Manual page 327

Chapter 13 Configuring Optional Spanning-Tree Features
Understanding Loop Guard
You can use loop guard to prevent alternate or root ports from becoming designated ports because of a
failure that leads to a unidirectional link. This feature is most effective when it is configured on the entire
switched network.
If your switch is running PVST or MSTP, you can enable this feature by using the spanning-tree
loopguard default global configuration command. The MSTP is available only if you have the EI
installed on your switch.
When the switch is operating in PVST mode, loop guard prevents alternate and root ports from becoming
designated ports, and spanning tree does not send BPDUs on root or alternate ports.
When the switch is operating in MST mode, BPDUs are not sent on nonboundary ports only if the port
is blocked by loop guard in all MST instances. On a boundary port, loop guard blocks the port in all MST
instances.
Configuring Optional Spanning-Tree Features
These sections describe how to configure optional spanning-tree features:
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78-14982-01
Default Optional Spanning-Tree Configuration, page 13-14
Enabling Port Fast, page 13-14
Enabling BPDU Guard, page 13-15
Enabling BPDU Filtering, page 13-16
Enabling UplinkFast for Use with Redundant Links, page 13-17
Enabling Cross-Stack UplinkFast, page 13-18
Enabling BackboneFast, page 13-19
Enabling Root Guard, page 13-19
Enabling Loop Guard, page 13-20
Configuring Optional Spanning-Tree Features
Catalyst 2950 Desktop Switch Software Configuration Guide
13-13