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Cisco ME 3400 Ethernet Access Switch Command Reference
OL-9640-10
Chapter 2 Cisco ME 3400 Ethernet Access Switch Cisco IOS Commands
spanning-tree loopguard default
spanning-tree loopguard default
Use the spanning-tree loopguard default global configuration command to enable loopguard by default
on all network node interfaces (NNIs) or enhanced network interface (ENIs) with STP enabled. Enabling
loopguard prevents 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 Spanning Tree Protocol (STP) is supported only on NNIs or on ENIs on which STP has been enabled.
To set a port as an NNI or ENI, enter the port-type {eni | nni} interface configuration command. To
enable STP on an ENI, enter the spanning-tree interface configuration command.
This command has no effect on user network interfaces (UNIs).
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.
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
12.2(25)EX This command was introduced.
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