Root guard influences root bridge selection by putting a designated port into an inconsistent state when it receives a superior bridge protocol data unit (BPDU).

Normally, a port that receives a superior BPDU will become the root port. However, if a designated port configured with root guard receives a superior BPDU, the port transitions to the root-inconsistent state and no data will flow through that port until it stops receiving superior BPDUs. As a result, root guard can be used to influence the placement of the root bridgeon a network by preventing other switches from propagating superior BPDUs throughout the network and becoming the root bridge.

Loop guard does not put a port into an inconsistent state when it receives a superior BPDU; loop guard puts a port into an inconsistent state when it stops receiving BPDUs. This prevents the trunk port from transitioning to the forwarding state, thereby preventing a Layer 2 switching loop.

BPDU guard does not put a port into an inconsistent state when it receives a superior BPDU; BPDU guard puts a port into the errdisable state when it receives any BPDUs. This isuseful for host ports, which should never receive BPDUs. BPDU guard defines the edge of the Spanning Tree Protocol (STP) domain by limiting the advertisement of BPDUs to a port.

PortFast does not put a port into an inconsistent state when it receives a superior BPDU; PortFast enables a port to immediately access the network by transitioning the port into the STP forwarding state without passing through the listening and learning states.