Cisco Catalyst 3550 Command Reference Manual page 443

Chapter 2 Cisco IOS Commands
Loop guard is most effective when it is configured on the entire switched network. 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 bridge protocol data units (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.
To disable root guard or loop guard, use the spanning-tree guard none interface configuration
command.
You can override the setting of the spanning-tree loopguard default global configuration command by
using the spanning-tree guard loop interface configuration command.
Examples
This example shows how to enable root guard on all the VLANs associated with the specified interface:
Switch(config)# interface fastethernet0/3
Switch(config-if)# spanning-tree guard root
This example shows how to enable loop guard on all the VLANs associated with the specified interface:
Switch(config)# interface fastethernet0/3
Switch(config-if)# spanning-tree guard loop
You can verify your settings by entering the show running-config privileged EXEC command.
Related Commands Command
show running-config
spanning-tree cost
spanning-tree loopguard default
spanning-tree mst cost
spanning-tree mst port-priority
spanning-tree mst root
spanning-tree port-priority
spanning-tree vlan
78-11195-09
Description
Displays the current operating configuration. For syntax
information, select Cisco IOS Configuration Fundamentals
Command Reference for Release 12.1 > Cisco IOS File
Management Commands > Configuration File Commands.
Sets the path cost for spanning-tree calculations.
Prevents alternate or root ports from becoming designated
ports because of a failure that leads to a unidirectional link.
Configures the path cost for MST calculations.
Configures an interface priority.
Configures the MST root switch priority and timers based on
the network diameter.
Configures an interface priority.
priority Sets the switch priority for the specified spanning-tree
instance.
Catalyst 3550 Multilayer Switch Command Reference
spanning-tree guard
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