Cisco Systems 4500 Enabling Loop Guard

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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 21 Configuring Optional STP Features Enabling Loop Guard
– If a channel is blocked by loop guard and the channel breaks, spanning tree loses all the state
information. The individual physical ports may obtain the forwarding state with the designated
role, even if one or more of the links that formed the channel are unidirectional.
Note You can enable UniDirectional Link Detection (UDLD) to help isolate the link failure.
A loop may occur until UDLD detects the failure, but loop guard is not able to detect it.
• Loop guard has no effect on a disabled spanning tree instance or a VLAN.
Enabling Loop Guard
You can enable loop guard globally or per-port.
To enable loop guard globally on the switch, perform this task:
This example shows how to enable loop guard globally:
Switch(config)# spanning-tree loopguard default
Switch(config)# Ctrl-Z
This example shows how to verify the previous configuration of port 4/4:
Switch# show spanning-tree interface fastethernet 4/4 detail
Port 196 (FastEthernet4/4) of VLAN0010 is forwarding
Port path cost 1000, Port priority 160, Port Identifier 160.196.
Designated root has priority 32768, address 00d0.00b8.140a
Designated bridge has priority 32768, address 00d0.00b8.140a
Designated port id is 160.196, designated path cost 0
Timers:message age 0, forward delay 0, hold 0
Number of transitions to forwarding state:1
The port is in the portfast mode by portfast trunk configuration
Link type is point-to-point by default
Bpdu filter is enabled
Loop guard is enabled by default on the port
BPDU:sent 0, received 0
To enable loop guard on an interface, perform this task:
Command Purpose
Step 1 Switch(config)# spanning-tree loopguard default Enables loop guard globally on the switch.
Step 2 Switch(config)# end Exits configuration mode.
Step 3 Switch# show spanning tree interface 4/4 detail Verifies the configuration impact on a port.
Command Purpose
Step 1 Switch(config)# interface {type slot/port} |
{port-channel port_channel_number}Selects an interface to configure.
Step 2 Switch(config-if)# spanning-tree guard loop Configures loop guard.
Step 3 Switch(config)# end Exits configuration mode.
Step 4 Switch# show spanning tree interface 4/4 detail Verifies the configuration impact on that port.