Users Guide
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 trac 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 trac 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 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.
All incoming and outgoing trac is blocked on an STP port in a root-inconsistent state. After the timeout period, the Switch C port
automatically transitions to a forwarding state as soon as device D stops sending BPDUs that advertise a lower priority.
If you enable a root guard on all STP ports on the links where the root bridge should not appear, you can ensure a stable STP
network topology and avoid bridging loops.
Figure 136. STP Root Guard Prevents Bridging Loops
Spanning Tree Protocol (STP)
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