Rapid Spanning Tree protocol (RSTP)

To further alleviate the 30 to 50 second convergence delays with STP, enhancements were made to the original IEEE 802.1D standard. The result was 802.1w, or Rapid Spanning Tree Protocol (RSTP RSTP is similar in many respects to STP. BPDU’s are forwarded between switches, and a Root Bridge is elected, based on the lowest Bridge ID. Root Ports and Designated Ports are also elected. RSTP defines five port types:

• Root Port – Switch port on each switch that has the best Path Cost to the Root Bridge (same as STP).
• Alternate Port – A backup Root Port, that has a less desirable Path Cost. An Alternate Port is placed in a discarding state.
• Designated Port – Non-Root port that represents the best Path Cost for each network segment to the Root Bridge (same as STP). Designated ports are also referred to as Point-to-Point ports.
• Backup Port – A backup Designated Port, that has a less desirable Path Cost. A Backup Port is placed in a discarding state.
• Edge Port – A port connecting a host device, which is moved to a Forwarding state immediately. If an Edge Port receives a BPDU, it will lose its Edge Port status and participate in RSTP calculations. On Cisco Catalyst switches, any port configured with PortFast becomes an Edge Port.

The key benefit of RSTP is speedier convergence. Switches no longer require artificial Forwarding Delay timers to ensure a loop-free environment. Switches instead perform a handshake synchronization to ensure a consistent topology table. During initial convergence, the Root Bridge and its directly-connected switches will place their interfaces in a discarding state. The Root Bridge and those switches will exchange BPDU’s, synchronize their topology tables, and then place their interfaces in a forwarding state. Each switch will then perform the same handshaking process with their downstream neighbors. The result is convergence that completes in a few seconds, as opposed to 30 to 50 seconds.

Changes to the RSTP topology are also handled more efficiently than 802.1D STP. Recall in that in 802.1D STP, a switch recognizing a topology change will send out a TCN (Topology Change Notification) BPDU, destined for the Root Bridge. Once the Root Bridge receives the TCN, it will send out a BPDU with the Topology Change (TC) bit set to all switches. When a switch receives this Root BPDU, it will temporarily lower its MAC-address Aging Timer from 300 seconds to 15 seconds, so that any erroneous MAC addresses can be quickly flushed out of the CAM table.
In RSTP, a switch recognizing a topology change does not have to inform the Root Bridge first. Any switch can generate and forward a TC BPDU. A switch receiving a TC BPDU will flush all MAC addresses learned on all ports, except for the port that received the TC BPDU. RSTP incorporates the features of UplinkFast by allowing Alternate and Backup ports to immediately enter a Forwarding state, if the primary Root or Designated port fails. RSTP also inherently employs the principles of BackboneFast, by not requiring an arbitrary Max Age timer for accepting inferior BPDU’s if there is an indirect network failure. 802.1w RSTP is backwards-compatible with 802.1D STP. However, when RSTP switches interact with STP switches, RSTP loses its inherent advantages, as will perform according to 802.1D specifications.

Two separate standards of RSTP have been developed:
• Rapid Per-VLAN Spanning Tree Protocol (RPVST+) – Cisco’s proprietary implementation of RSTP.
• Multiple Spanning Tree (MST) – The IEEE 802.1s standard or RSTP.