In STP topology 3 (shown in the lower middle), if you have enabled the root guard feature on the STP port on Switch C thatconnects to device D, and device D sends a superior BPDU that would trigger the election of device D as the new root bridge, theBPDU is ignored and the port on Switch C transitions from a forwarding to a root-inconsistent state (shown by the green X icon). Asa result, Switch A becomes the root bridge.Figure 131. STP Root Guard Prevents Bridging LoopsConfiguring Root GuardEnable 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.• Root guard is supported on a port in any Spanning Tree mode:– Spanning Tree Protocol (STP)– Rapid Spanning Tree Protocol (RSTP)– Multiple Spanning Tree Protocol (MSTP)– Per-VLAN Spanning Tree Plus (PVST+)• When enabled on a port, root guard applies to all VLANs configured on the port.• You cannot enable root guard and loop guard at the same time on an STP port. For example, if you configure root guard on a porton which loop guard is already configured, the following error message displays: • % Error: LoopGuard isconfigured. Cannot configure RootGuard.• When used in an MSTP network, if root guard blocks a boundary port in the CIST, the port is also blocked in all other MSTinstances.To enable the root guard on an STP-enabled port or port-channel interface in instance 0, use the following command.• Enable root guard on a port or port-channel interface.INTERFACE mode or INTERFACE PORT-CHANNEL modeSpanning Tree Protocol (STP) 853
