What is RSTP (Rapid Spanning Tree Protocol)?

RSTP (Rapid Spanning Tree Protocol) is an evolution of STP. It was originally introduced as IEEE 802.1w standard and in 2004 IEEE decided to replace STP with RSTP in 802.1D standard. Finally, in 2011, in the IEEE decided to move all the RSTP details into 802.1Q standard.

RSTP is backwards-compatible with STP and there are many similarities between the two protocols, such as:

  • the root switch is elected using the same set of rules in both protocols
  • root ports are selected with the same rules, as well as designated port on LAN segments
  • both STP and RSTP place each port in either forwarding or blocking state. The blocking state in RSTP is called the discarding state.

However, there are differences between STP and RSTP:

  • RSTP enables faster convergence times than STP (usually within just a couple of seconds)
  • STP ports states listening, blocking, and disabled are merged into a single state in RSTP – the discarding state
  • STP features two port types – root and designated port. RSTP adds two additional port types – alternate and backup port.
  • with STP, the root switch generates and sends Hellos to all other switches, which are then relayed by the non-root switches. With RSTP, each switch can generate its own Hellos.

Consider the following network topology with RSTP turned on:

RSTP example topology

In order to avoid loops, RSTP has placed one port on SW3 in the alternate state. This port will not process or forward any frames except the RSTP messages. However, if the root port on SW3 fails, the alternate port will rapidly become the root port and start forwarding frames.

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