What happens when a superior BPDU is received on a port configured for Root Guard?

When a superior BPDU is received on a port that is configured for Root Guard, the port is placed in an ErrDisabled state. Root Guard is a feature used in Spanning Tree Protocol (STP) to ensure that only designated root bridges can be elected by preventing other switches from claiming root bridge status. When a superior BPDU—indicating that a switch is attempting to become the root bridge—arrives on the port, Root Guard interprets this as a violation of the expectations for that port. Consequently, to maintain network stability and prevent an undesired topology change, the port is disabled and placed in an ErrDisabled state. This action helps in preserving the integrity of the spanning tree by halting any potential disruptions that could arise from an unexpected root bridge election.

The other outcomes like forwarding traffic immediately or entering the Forwarding state would contradict the purpose of Root Guard, which aims to protect the root bridge configuration. Additionally, resetting to default configurations does not relate to how Root Guard is designed to operate in response to superior BPDUs.