Dell S4048T, Configuration Manual

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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.
Figure 130. STP Root Guard Prevents Bridging Loops
Spanning Tree Protocol (STP) 1056