Cisco ME 3600X, Command Reference Manual

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Cisco ME 3800X and ME 3600X Switch Command Reference
OL-28238-01
Chapter
spanning-tree loopguard default
spanning-tree loopguard default
To enable loopguard by default on all interfaces with STP enabled, use the spanning-tree loopguard
default command in global configuration mode. Enabling loopguard prevents alternate or root ports
from becoming designated ports because of a failure that leads to a unidirectional link. To return to the
default setting, use the
no form of this command.
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 interfaces if the
interface is blocked by loop guard in all MST instances. On a boundary interface, loop guard blocks the
interface in all MST instances.
Loop guard operates only on STP ports that the spanning tree identifies as point-to-point.
You can override the setting of the spanning-tree loopguard default global configuration command by
using the spanning-tree guard loop interface configuration command.
You can verify your settings by entering the show running-config privileged EXEC command.
Examples
This example shows how to globally enable loop guard:
Switch(config)# spanning-tree loopguard default
Release Modification
12.252)EY This command was introduced.