Spanning-Tree Loopguard Default - Cisco Catalyst 2950 Command Reference Manual

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.
Syntax Description
This command has no arguments or keywords.
Defaults
Loop guard is disabled.
Command Modes
Global configuration
Command History Release
12.1(9)EA1
Usage Guidelines
You can enable the loop guard feature when the switch is operating in the per-VLAN spanning-tree
(PVST) or the multiple spanning-tree (MST) mode. The MST mode is available only if you have the
enhanced software image (EI) installed on your switch.
Loop guard is most effective when it is configured on the entire switched network. When the switch is
operating in 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.
Catalyst 2950 Desktop Switch Command Reference
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spanning-tree loopguard default
no spanning-tree loopguard default
Modification
This command was first introduced.
Chapter 2 Cisco IOS Commands
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