28.10.4 Configuring Loop Guard

When a unidirectional link fault occurs, such as when a one-way line is cut, and BPDU reception is cut off, a loop might have occurred. Loop guard functionality prevents these kinds of loops from occurring.

Points to note

Loop guards run on ports for which PortFast functionality is not set.

When the spanning-tree loopguard default command is set, loop guards are applied to all ports other than those with PortFast set. When this is applied by default, the spanning-tree guard none command is set to disable loop guards.

Command examples

1. (config)# spanning-tree loopguard default

   Sets that loop guard functionality is to be applied for all ports other than those with PortFast set.

2. (config)# interface gigabitethernet 1/0/1

   (config-if)# spanning-tree guard none

   (config-if)# exit

   Sets loop guards to be disabled on port 1/0/1 when loop guards are set to be applied by default.

3. (config)# no spanning-tree loopguard default

   (config)# interface gigabitethernet 1/0/2

   (config-if)# spanning-tree guard loop

   Deletes settings to apply loop guards by default, and applies loop guards by setting for each port, for port 1/0/2. Also, loop guard is applied with per-port settings for ports 1, 0, and 2.