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How Does RSTP Improve Upon STP?


The IEEE defines STP in the 802.1d IEEE standard. The IEEE has improved the 802.1d protocol with the definition of Rapid Spanning Tree Protocol (RSTP), as defined in standard 802.1w.

RSTP (802.1w) works just like STP (802.1d) in several ways:



RSTP can be deployed alongside traditional 802.1d STP switches, with RSTP features working in switches that support it, and traditional 802.1d STP features working in the switches that support only STP.

STP takes a relatively long time to converge (50 seconds with the default settings). RSTP improves network convergence when topology changes occur.

RSTP improves convergene by either eliminating or significantly reducing the waiting periods that 802.1d STP needs to avoid loops during convergence. 802.1d STP requires a waiting period of MaxAge (default 20 seconds) before reacting to some events, whereas RSTP only has to wait 3*Hello (default 6 seconds). Additionally, RSTP eliminates the forward delay (default 15 seconds) time in both Listening and Learning States. Traditional STP convergence has essentially three time periods each of which RSTP improves upon. These three waiting periods of (by default) 20, 15, and 15 seconds create 802.1d STP's relatively slow convergence, and the reduction or elimination of these waiting periods makes RSTP convergence occur quickly.







References

Odom, Wendell (2008) CCNA ICND2 Official Exam Certification Guide, Second Edition. Indianapolis: Cisco Press.