Reference Guide

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.
All incoming and outgoing traffic 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 133. STP Root Guard Prevents Bridging Loops
Configuring Root Guard
Enable STP root guard on a per-port or per-port-channel basis.
Dell Networking OS Behavior: The following conditions apply to a port enabled with STP root guard:
• Root guard is supported on any STP-enabled port or port-channel interface except when used as a
stacking port.
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Spanning Tree Protocol (STP)