Concept Guide

DellEMC(conf-if-gi-1/7)#do show ip interface brief gigabitEthernet 1/7
Interface IP-Address OK Method Status Protocol
GigabitEthernet 1/7 unassigned YES Manual up up
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 species a bridge priority of 8192.
The secondary option species 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.
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 ow 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 trac then begins to ow 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 trac ow, 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
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Spanning Tree Protocol (STP)