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Catalyst 2950 and Catalyst 2955 Switch Command Reference
0L-10102-01
Chapter 2 Catalyst 2950 and 2955 Cisco IOS Commands
spanning-tree loopguard default
spanning-tree loopguard default
Use the spanning-tree loopguard default global configuration command to prevent alternate or root
ports from becoming designated ports because of a failure that leads to a unidirectional link. Use the no
form of this command to return to the default setting.
spanning-tree loopguard default
no spanning-tree loopguard default
Syntax Description This command has no arguments or keywords.
Defaults Loop guard is disabled.
Command Modes Global configuration
Command History
Usage Guidelines You can enable the loop guard feature when the switch is operating in the per-VLAN spanning-tree plus
(PVST+), rapid-PVST+, or the multiple spanning-tree (MST) mode.
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.
Loop guard operates only on ports that are considered point-to-point by the spanning tree.
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 globally enable loop guard:
Switch(config)# spanning-tree loopguard default
You can verify your settings by entering the show running-config privileged EXEC command.
Release Modification
12.1(9)EA1 This command was introduced.
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