Hello Sathish
Traditional STP relies on timers (Hello, Forward Delay, listening, learning, and Max Age) to manage the convergence of the spanning tree. As you correctly insinuated, RSTP introduces a more dynamic approach to achieve faster convergence using its sync mechanism.
While this sync mechanism used by RSTP minimizes the reliance on traditional timers for convergence, it still uses a few timers for specific purposes. One of these is the proposal timer.
When a switch wants to initiate a change in the STP topology (for example, when a new switch is added or a topology change is detected), it sends a Proposal BPDU to its neighboring switches. The Proposal Timer helps manage this process by:
- Timing the Response: The Proposal Timer sets a limit on how long a switch should wait for an Agreement BPDU from its neighbors after it sends out a Proposal BPDU.
- Avoiding Loops: By using this timer, RSTP ensures that switches do not immediately transition their ports to the forwarding state, which could potentially create temporary loops in the network. Instead, they wait for an explicit agreement from their neighbors.
The proposal timer in this case is not used like the listening, learning, and forward delay timers in traditional STP, where they must elapse before the topology is functional. It is a timeout timer that is used as part of the sync mechanism. Does that make sense?
I hope this has been helpful!
Laz