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17-9
Catalyst 2960 Switch Software Configuration Guide
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Chapter 17 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 enabled on the entire
switched network. Loop guard prevents alternate and root ports from becoming designated ports, and
spanning tree does not send BPDUs on root or alternate ports.
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
interface is blocked by loop guard in all MST instances. On a boundary port, loop guard blocks the
interface in all MST instances.
Configuring Optional Spanning-Tree Features
These sections contain this configuration information:
• Default Optional Spanning-Tree Configuration, page 17-9
• Optional Spanning-Tree Configuration Guidelines, page 17-10
• Enabling Port Fast, page 17-10 (optional)
• Enabling BPDU Guard, page 17-11 (optional)
• Enabling BPDU Filtering, page 17-12 (optional)
• Enabling UplinkFast for Use with Redundant Links, page 17-13 (optional)
• Enabling BackboneFast, page 17-14 (optional)
• Enabling EtherChannel Guard, page 17-14 (optional)
• Enabling Root Guard, page 17-15 (optional)
• Enabling Loop Guard, page 17-15 (optional)
Default Optional Spanning-Tree Configuration
Table 17-1 shows the default optional spanning-tree configuration.
Table 17-1 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.
BackboneFast Globally disabled.
EtherChannel guard Globally enabled.
Root guard Disabled on all interfaces.
Loop guard Disabled on all interfaces.