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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

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 16-5
• Optional Spanning-Tree Configuration Guidelines, page 16-6
• Enabling Port Fast, page 16-6 (optional)
• Enabling BPDU Guard, page 16-7 (optional)
• Enabling BPDU Filtering, page 16-8 (optional)
• Enabling EtherChannel Guard, page 16-9 (optional)
• Enabling Root Guard, page 16-10 (optional)
• Enabling Loop Guard, page 16-10 (optional)

Default Optional Spanning-Tree Configuration

Table 16-1 shows the default optional spanning-tree configuration. Only NNIs or ENIs with STP enabled
participate in STP on the switch. UNIs and ENIs that have not been configured for STP are always in the
forwarding state.
Tab le 16-1 Default Optional Spanning-Tree Configuration
Feature Default Setting
Port Fast, BPDU filtering, BPDU guard Globally disabled (unless they are individually configured per STP port).
EtherChannel guard Globally enabled.
Root guard Disabled on all STP ports.
Loop guard Disabled on all STP ports.