Understanding Loop Guard; Configuring Optional Spanning-Tree Features - Cisco Catalyst 2950 Software Configuration Manual

Chapter 12

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 enhanced
software image 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 include optional spanning-tree configuration information:
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78-11380-04
Default Optional Spanning-Tree Configuration, page 12-14
Enabling Port Fast, page 12-14
Enabling BPDU Guard, page 12-15
Enabling BPDU Filtering, page 12-16
Enabling UplinkFast for Use with Redundant Links, page 12-17
Enabling Cross-Stack UplinkFast, page 12-18
Enabling BackboneFast, page 12-19
Enabling Root Guard, page 12-19
Enabling Loop Guard, page 12-20
Configuring Optional Spanning-Tree Features
Catalyst 2950 Desktop Switch Software Configuration Guide
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