Users Guide
Figure 135. BPDU Filtering Enabled on an Interface
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 species a bridge priority of 8192.
The secondary option species 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.
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Spanning Tree Protocol (STP)