Reference Guide

Selecting STP Root
STP determines the root bridge, but you can assign one bridge a lower priority to increase the likelihood
that it becomes the root bridge. You can also specify that a bridge is the root or the secondary root.
To change the bridge priority or specify that a bridge is the root or secondary root, use the following
command.
• Assign a number as the bridge priority or designate it as the root or secondary root.
PROTOCOL SPANNING TREE mode
bridge-priority {priority-value | primary | secondary}
– priority-value: the range is from 0 to 65535. The lower the number assigned, the more likely
this bridge becomes the root bridge.
The primary option specifies a bridge priority of 8192.
The secondary option specifies a bridge priority of 16384.
The default is 32768.
Example of Viewing STP Root Information
To view only the root information, use the show spanning-tree root command from EXEC privilege
mode.
Dell#show spanning-tree 0 root
Root ID Priority 32768, Address 0001.e80d.2462
We are the root of the spanning tree
Root Bridge hello time 2, max age 20, forward delay 15
Dell#
STP Root Guard
To avoid bridging loops, use the STP root guard feature in a Layer 2 network. In STP, the switch in the
network with the lowest priority (as determined by STP or set with the bridge-priority command) is
selected as the root bridge.
If two switches have the same priority, the switch with the lower MAC address is selected as the root. All
other switches in the network use the root bridge as the reference used to calculate the shortest
forwarding path.
Because any switch in an STP network with a lower priority can become the root bridge, the forwarding
topology may not be stable. The location of the root bridge can change, resulting in unpredictable
network behavior. The STP root guard feature ensures that the position of the root bridge does not
change.
Root Guard Scenario
For example, as shown in the following illustration (STP topology 1, upper left) Switch A is the root bridge
in the network core. Switch C functions as an access switch connected to an external device. The link
between Switch C and Switch B is in a Blocking state. The flow of STP BPDUs is shown in the illustration.
In STP topology 2 (shown in the upper right), STP is enabled on device D on which a software bridge
application is started to connect to the network. Because the priority of the bridge in device D is lower
Spanning Tree Protocol (STP)
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