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15-13
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 15 Configuring Optional Spanning-Tree Features
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.
You can enable this feature by using the spanning-tree loopguard default global configuration
command.
When the switch is operating in PVST+ or rapid-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:
• Default Optional Spanning-Tree Configuration, page 15-13
• Optional Spanning-Tree Configuration Guidelines, page 15-14
• Enabling Port Fast, page 15-14 (optional)
• Enabling BPDU Guard, page 15-15 (optional)
• Enabling BPDU Filtering, page 15-16 (optional)
• Enabling UplinkFast for Use with Redundant Links, page 15-17 (optional)
• Enabling Cross-Stack UplinkFast, page 15-18 (optional)
• Enabling BackboneFast, page 15-19 (optional)
• Enabling EtherChannel Guard, page 15-19 (optional)
• Enabling Root Guard, page 15-20 (optional)
• Enabling Loop Guard, page 15-20 (optional)
Default Optional Spanning-Tree Configuration
Table 15-1 shows the default optional spanning-tree configuration.
Ta b l e 15-1 Default Optional Spanning-Tree Configuration
Feature Default Setting
Port Fast, BPDU
filtering, BPDU guard
Globally disabled (unless they
are individually configured
per interface).
UplinkFast Globally disabled.
CSUF Disabled on all interfaces.
BackboneFast Globally disabled.
EtherChannel guard Globally enabled.