Similarities and Differences between STP and RSTP

The IEEE originally developed the STP protocol to prevent network loops and later introduced RSTP to address modern network needs. This tutorial compares STP and RSTP, focusing on their similarities and differences.

RSTP is an updated version of STP and uses the same framework. Both protocols can operate within the same network. RSTP features are available on compatible switches, while traditional STP features remain active on switches that only support STP.

Similarities between RSTP and STP

The following points outline the similarities between STP and RSTP.

• Both run a single STP instance on the switch.
• Both create and use a virtual topology to find switching loops in the network.
• Both elect a root bridge and use it as the focal point of the virtual topology.
• Both use the same rules and tiebreakers for the root bridge election.
• In both protocols, all switches other than the root bridge become non-root bridge switches.
• Both select only one root port to reach the root bridge.
• Both use the same rules and tiebreakers to select the root port.
• Both select a single designated port for each local segment or end device.
• Both use the same rules and tiebreakers to select the designated port.
• After selecting the root and designated ports, both protocols block all remaining ports to eliminate loops.
• Both STP and RSTP transition ports through several states. During these states, they learn the network topology, flush CAM table entries, select root and designated ports, and block ports that cause loops.

Differences between RSTP and STP

The primary motivation for RSTP was to improve convergence, which is the time a protocol takes to respond to network changes. STP typically converges in approximately 50 seconds by default. RSTP significantly reduces this time, typically converging within a few seconds or, in slower scenarios, up to 10 seconds.

RSTP introduces two new port types to enable rapid convergence: alternative and backup ports. The alternative port provides a path to the root bridge if the root port link fails, while the backup port offers access to the local segment if the designated port link fails.

With alternative and backup ports, RSTP does not need to reselect root or designated ports during network changes. If the root path fails, the alternative port is activated within seconds. If the designated path fails, the backup port is enabled. As a result, networks using RSTP typically experience a maximum delay of 10 seconds during changes.

STP does not support alternative or backup ports. If the root or designated path fails, the protocol must rerun the selection process, which takes approximately 50 seconds. Therefore, networks using STP experience a 50-second delay during changes.

RSTP eliminates the learning state and renames the blocking state as the discarding state, combining the disabled and blocking states under this new designation. In the disabled state, the port can work physically, but it can not forward user frames. It simply meant that the interface was administratively disabled.

STP and RSTP 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 does not wait for this time to expire. It flashes all CAM table entries whenever a change occurs.

Conclusion

STP and RSTP share many foundational concepts and operational similarities. RSTP introduces critical improvements to address STP's limitations, particularly in convergence speed and network resilience. The enhancements provided by RSTP (such as alternative and backup ports, faster port transitions, and improved handling of network changes) make it a more robust solution for modern networks. Therefore, understanding both protocols and their differences is essential for effective network design and management.