Cisco Systems 4500 About Root Guard, Enabling Root Guard

21-2
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 21 Configuring Optional STP Features
About Root Guard
Note For complete syntax and usage information for the switch commands used in this chapter, look at the
Cisco Catalyst 4500 Series Switch Command Reference and related publications at this location:
http://www.cisco.com/en/US/products/hw/switches/ps4324/index.html
If the command is not found in the Catalyst 4500 Command Reference, it is located in the larger Cisco
IOS library. Refer to the Cisco IOS Command Reference and related publications at this location:
http://www.cisco.com/en/US/products/ps6350/index.html
About Root Guard
Spanning Tree root guard forces an interface to become a designated port, to protect the current root
status and prevent surrounding switches from becoming the root switch.
When you enable root guard on a per-port basis, it is automatically applied to all of the active VLANs
to which that port belongs. When you disable root guard, it is disabled for the specified port and the port
automatically goes into the listening state.
When a switch that has ports with root guard enabled detects a new root, the ports enter the
root-inconsistent state. The switch no longer detects a new root and its ports automatically go into the
listening state.
Enabling Root Guard
To enable root guard on a Layer 2 access port (to force it to become a designated port), perform this task:
This example shows how to enable root guard on Fast Ethernet interface 5/8:
Switch(config)# interface fastethernet 5/8
Switch(config-if)# spanning-tree guard root
Switch(config-if)# end
Switch#
This example shows how to verify the configuration:
Switch# show running-config interface fastethernet 5/8
Building configuration...
Current configuration: 67 bytes
!
Command Purpose
Step 1 Switch(config)# interface {{fastethernet |
gigabitethernet | tengigabitethernet} slot/port}Specifies an interface to configure.
Step 2 Switch(config-if)# [no] spanning-tree guard root Enables root guard.
Use the no keyword to disable root guard.
Step 3 Switch(config-if)# end Exits configuration mode.
Step 4 Switch# show spanning-tree Verifies the configuration.