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RSTP - Proposal Agreement Process Simplified

 In RSTP (Rapid Spanning Tree Protocol), the Proposal and Agreement process is key to optimizing convergence times when a topology change occurs. Here’s how it works:

  1. Proposal Stage:

    • When a switch detects a link coming up (such as when a new switch is connected or a port goes from blocking to forwarding), it assumes it could become the designated bridge on that port.
    • The switch sends a Proposal BPDU on that port, which signals to the downstream switch that it intends to make the port a designated port and immediately start forwarding traffic.

  2. Agreement Stage:

    • The downstream switch (receiving the Proposal BPDU) checks whether the port on its side should be in blocking or forwarding state.
    • If the downstream switch agrees with the proposal and can allow the new topology without creating loops, it sends an Agreement BPDU back to the original switch.
    • Upon receiving the Agreement, the original switch places the port into the Forwarding state immediately without waiting for the long transition times used by legacy STP (802.1D).

  3. Synchronization:

    • The downstream switch will block its non-edge ports until it ensures that they will not form loops, i.e., until it completes its own Proposal and Agreement process with other switches further downstream.

Key Advantages:

  • Rapid Convergence: The Proposal and Agreement mechanism allows rapid transitions to the forwarding state without the long delays experienced in legacy STP.
  • Loop-Free Topology: This process ensures that no loops are formed during topology changes because switches only allow forwarding when they agree on the new topology.

In essence, RSTP achieves much faster convergence compared to legacy STP by utilizing this Proposal and Agreement mechanism, which replaces the slow transitional states of Listening and Learning.

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