Cisco Systems 3750E manual Understanding Loop Guard, 20-11

Chapter 20 Configuring Optional Spanning-Tree Features

Configuring Optional Spanning-Tree Features

Figure 20-9	Root Guard in a Service-Provider Network
	Customer network	Service-provider network
Potential
spanning-tree root without
root guard enabled			Desired
			root switch

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.

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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 20-12

•Optional Spanning-Tree Configuration Guidelines, page 20-12

•Enabling Port Fast, page 20-12(optional)

•Enabling BPDU Guard, page 20-13(optional)

•Enabling BPDU Filtering, page 20-14(optional)

•Enabling UplinkFast for Use with Redundant Links, page 20-15(optional)

•Enabling Cross-Stack UplinkFast, page 20-16(optional)

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02		20-11