Chapter 19
| Spanning Tree Commands
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Related Commands
spanning-tree cost (527)
spanning-tree
root-guard
This command prevents a designated port from taking superior BPDUs into
account and allowing a new STP root port to be elected. Use the no form to disable
this feature.
Syntax
[no] spanning-tree root-guard
Default Setting
Disabled
Command Mode
Interface Configuration (Ethernet, Port Channel)
Command Usage
â—† A bridge with a lower bridge identifier (or same identifier and lower MAC
address) can take over as the root bridge at any time.
â—† When Root Guard is enabled, and the switch receives a superior BPDU on this
port, it is set to the Discarding state until it stops receiving superior BPDUs for a
fixed recovery period. While in the discarding state, no traffic is forwarded
across the port.
â—† Root Guard can be used to ensure that the root bridge is not formed at a
suboptimal location. Root Guard should be enabled on any designated port
connected to low-speed bridges which could potentially overload a slower link
by taking over as the root port and forming a new spanning tree topology. It
could also
be used to form a border around part of the network where the root
bridge is allowed.
â—† When spanning tree is initialized globally on the switch or on an interface, the
switch will wait for 20 seconds to ensure that the spanning tree has converged
before enabling Root Guard.
Example
Console(config)#interface ethernet 1/5
Console(config-if)#spanning-tree edge-port
Console(config-if)#spanning-tree root-guard
Console(config-if)#
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