Chapter 18
| Spanning Tree Commands
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Command Usage
◆ A port connecting a LAN through the bridge to the root bridge is known as a
designated port. A bridge with a designated port and 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
could also
be used to form a border around part of the network where the root
bridge is allowed.
◆ 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)#
spanning-tree
spanning-disabled
This command disables the spanning tree algorithm for the specified interface. Use
the no form to re-enable the spanning tree algorithm for the specified interface.
Syntax
[no] spanning-tree spanning-disabled
Default Setting
Enabled
Command Mode
Interface Configuration (Ethernet, Port Channel)
Example
This example disables the spanning tree algorithm for port 5.
Console(config)#interface ethernet 1/5
Console(config-if)#spanning-tree spanning-disabled
Console(config-if)#
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