IBM 12.1(22)EA6 User Manual

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.

These sections describe how to configure optional spanning-tree features:

 (optional)

 (optional)

 (optional)

 (optional)

 (optional)

 (optional)

Customer network

Potential

spanning-tree root without

root guard enabled

Enable the root-guard feature
on these interfaces to prevent
switches in the customer
network from becoming
the root switch or being
in the path to the root.

Desired

root switch

Service-provider network

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