STP, RSTP, PVST, RPVST, and MSTP

A long time ago, Digital Equipment Corporation (DEC) created the original version of STP as a proprietary protocol for its switches. Later, IEEE developed an open-standard version of STP. Although it runs on all vendors' switches, it has several limitations. It runs only one instance per switch. It also has slow convergence. To overcome these issues, Cisco developed its own STP. Cisco's STP is based on the IEEE standard. It uses the same terminology and concepts but provides some additional features. It runs one instance per VLAN. Cisco's STP is called PVST (Per VLAN Spanning-Tree).

Later, IEEE updated the STP to RSTP to meet the requirements of modern networks. RSTP is the updated version of STP. It speeds up the convergence process. It uses the same framework STP uses. You can use both in the same network. RSTP features will work in switches that support it, and traditional STP features will work in switches that support only STP.

Cisco also updated its variation with RPVST. RPVST is based on RSTP. Similar to PVST, it uses the same terminology and concepts as RSTP, supports one instance per VLAN, and provides additional features such as PortFast and EtherChannel.

The main issue with IEEE's STP and RSTP is that they both run only one instance per switch, which does not scale well for large networks running several VLANs. To support VLANs, IEEE created a special version of STP, called MSTP. MSTP runs one instance per VLAN.

Similarities between STP, RSTP, PVST, RPVST, and MSTP

The following are the similarities between STP, RSTP, PVST, RPVST, and MSTP.

• All create and use a virtual topology to find switching loops in every VLAN.
• All elect a root bridge and use it as the focal point of the virtual topology.
• All use the same rules and tiebreakers for the root bridge election.
• Besides the root bridge, all remaining switches become non-root bridge switches.
• All select only one root port to reach the root bridge.
• All use the same rules and tiebreakers to select the root port.
• All select only one designated port for every local segment or end device.
• All use the same rules and tiebreakers to select the designated port.
• After selecting the root and designated ports, they block all remaining ports to remove loops.
• All variations run all ports through a few states. During these states, they learn the network topology, flush CAM table entries, select root and designated ports, and block the ports causing the loops.

Differences between STP, RSTP, PVST, RPVST, and MSTP

The main reason for creating RSTP/RPVST was the convergence. Convergence is the time it takes STP to respond to a network change. STP/PVST takes a relatively long time for convergence. With default settings, it takes 50 seconds to reach convergence. RSTP/RPVST/MSTP improves it. It converges within a few seconds (or, in slow conditions, within about 10 seconds).

RSTP/RPVST/MSTP adds two new port types for rapid convergence. These port types are alternative and backup ports. It uses the alternative port to reach the root bridge if the link connected to the root port fails. It uses the backup port to reach the local segment when the link connected to the designated port fails.

Since RSTP/RPVST/MSTP has an alternative or backup port, it does not have to reselect the root or designated ports when a network change occurs. If the root path fails, it enables the alternative port within a few seconds. If the designated path fails, the backup port is enabled. It means that if the network is running RSTP/RPVST/MSTP, it must wait up to 10 seconds whenever a change occurs.

STP/PVST does not support the concept of alternative or designated ports. If the root or designated path fails, the process reruns to select a new one. Running the entire process again takes 50 seconds. This means that if a network is running STP, it must wait 50 seconds after every change.

RSTP/RPVST/MSTP does not use the learning state. It also renames the blocking state to the discarding state and slightly redefines its use. It combines disabled and blocking states into the discarding state. In the disabled state, the port can still work physically, but it cannot forward user frames. It simply meant that the interface was administratively disabled.

The following table compares STP/PVST and RSTP/RPVST/MSTP port states.

STP/PVST and RSTP/RPVST/MSTP flush CAM table entries differently. Each CAM table entry has a default age-old time. Once this time has expired, the switch removes the entry from the CAM table. STP waits for this time to expire. RSTP/RPVST/MSTP does not wait for this time to expire. It flashes all CAM table entries whenever a change occurs.

Comparing STP, RSTP, PVST, RPVST, and MSTP

Conclusion

Evolution of spanning tree protocols (from STP to RSTP, PVST to RPVST, and MSTP) reflects the need for faster convergence, better scalability, and enhanced support for VLANs in increasingly complex networks. Each protocol builds on its predecessor by introducing improvements in efficiency and flexibility while maintaining core concepts such as loop prevention and root bridge election. By understanding the similarities and differences among these protocols, you can choose the most appropriate solution to ensure robust, stable, and high-performing network topologies.