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Selecting STP Root
The 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.
To view only the root information, use the show spanning-tree root command from EXEC privilege mode.
DellEMC#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
DellEMC#
STP Root Guard
Use the STP root guard feature in a Layer 2 network to avoid bridging loops. 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 than the root bridge in Switch A, device D is
elected as root, causing the link between Switches A and B to enter a Blocking state. Network traffic then begins to flow in the
directions indicated by the BPDU arrows in the topology. If the links between Switches C and A or Switches C and B cannot
handle the increased traffic flow, frames may be dropped.
In STP topology 3 (shown in the lower middle), if you have enabled the root guard feature on the STP port on Switch C that
connects to device D, and device D sends a superior BPDU that would trigger the election of device D as the new root bridge,
the BPDU is ignored and the port on Switch C transitions from a forwarding to a root-inconsistent state (shown by the green X
icon). As a result, Switch A becomes the root bridge.
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Spanning Tree Protocol (STP)