Why RSTP Is Required

As the scale of LANs increases continuously, the convergence speed of STP is too slow and cannot meet service requirements. In 2001, IEEE introduced the Rapid Spanning Tree Protocol (RSTP), which is standardized as IEEE 802.1w. RSTP was developed based on STP and makes additions and modifications to STP.

The STP convergence speed is slow due to the following:

• STP waits until the timer expires before determining changes in the topology. This slows down network convergence.
• STP requires the root bridge to send configuration BPDUs after the network topology becomes stable and requires other devices to process and spread the configuration BPDUs to the entire STP network.

STP does not clearly distinguish port states or port roles. Ports in Listening, Learning, and Blocking states appear the same to users because these ports are all prevented from forwarding service traffic. In terms of port use and configuration, the main differences between ports are found in the port roles rather than port states.

RSTP makes the following improvements over STP:

• RSTP removes three port states, defines two new port roles, and decouples port states from port roles. It changes the configuration BPDU format and uses the Flags field to describe port roles.
• RSTP processes configuration BPDUs differently from STP.
  • RSTP allows non-root bridges to send configuration BPDUs at Hello timer intervals after the topology becomes stable, regardless of whether they have received configuration BPDUs from the root bridge.
  • A device determines that the negotiation between its port and the upstream device has failed if the port does not receive any configuration BPDUs sent from the upstream device within the timeout interval (Hello timer x 3 x Timer Factor). Because the device does not have to wait for a Max Age period as is required in STP, the timeout interval is shorter.
  • When an RSTP port receives an RST BPDU from the upstream designated bridge, the port compares the received RST BPDU with its own RST BPDU. If its RST BPDU is superior to the received one, the port discards the received RST BPDU and immediately responds to the upstream device with its own RST BPDU. After receiving the RST BPDU, the upstream device replaces its RST BPDU with the received RST BPDU. This allows RSTP to rapidly process inferior BPDUs without relying on timers.
• RSTP introduces rapid convergence mechanisms, including the Proposal/Agreement mechanism, fast switchover of the root port, and edge port.
• RSTP adds multiple protection functions, including BPDU protection, root protection, loop protection, and TC BPDU attack defense.