CLI Reference Guide-R07

Chapter 19 | Spanning Tree Commands
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Command Usage
◆ This command defines the priority for the use of a port in the Spanning Tree
Algorithm. If the path cost for all ports on a switch are the same, the port with
the highest priority (that is, lowest value) will be configured as an active link in
the spanning tree.
◆ Where more than one port is assigned the highest priority, the port with lowest
numeric identifier will be enabled.
◆ The criteria used for determining the port role is based on root bridge ID, root
path cost, designated bridge, designated port, port priority, and port number,
in that order and as applicable to the role under question.
Example
Console(config)#interface ethernet 1/5
Console(config-if)#spanning-tree port-priority 0
Related Commands
spanning-tree cost (461)
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
◆ A bridge with a lower bridge identifier (or same identifier and lower MAC
address) can take over as the root bridge at any time.
◆ 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.
◆ 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