How many port states are involved in RSTP?

Rapid Spanning Tree Protocol (RSTP), defined in IEEE 802.1w, optimizes the original Spanning Tree Protocol (STP) by reducing the number of port states to accelerate network convergence. RSTP consolidates the STP states of Disabled, Blocking, and Listening into a single 'Discarding' state. The Learning and Forwarding states remain functionally the same. Consequently, RSTP operates with only three port states: Discarding, Learning, and Forwarding. This streamlined state machine allows RSTP-enabled ports to transition to the Forwarding state much more quickly than in legacy STP.

A: Incorrect. RSTP requires multiple states to manage loop prevention, MAC address learning, and data forwarding functions. A single state would be insufficient. B: Incorrect. Two states are not enough to differentiate between the phase of learning MAC addresses and the phase of actively forwarding user traffic. C: Incorrect. Four states are more aligned with legacy STP (Blocking, Listening, Learning, Forwarding), which RSTP was designed to improve upon by simplifying the states.

1. Huawei. (2020). HCIA-Datacom V1.0 Training Material. H12-811, Section "RSTP Principles", pp. 218. "RSTP defines three port states: Discarding, Learning, and Forwarding."

2. IEEE Std 802.1D-2004. (2004). IEEE Standard for Local and metropolitan area networks—Media Access Control (MAC) Bridges. Section 17.4 "Port states", p. 153. This standard, which incorporates 802.1w, states, "The port states are DISCARDING, LEARNING, and FORWARDING."