STP - RSTP Proposal Timer Operation

STP - RSTP Proposal Timer Operation

Tags: #stp #rstp #proposal #timer

In the Rapid Spanning Tree Protocol (RSTP) version of STP, when a designated port sends a proposal but does not receive a Bridge Protocol Data Unit (BPDU) with the agreement bit set, the following actions occur:

1. The designated port starts a proposal timer to allow the downstream switch time to process the proposal and respond with an agreement BPDU.
2. Until the agreement is received, the port that sent the proposal will not transition to the forwarding state, preventing network loops during topology changes.
3. The designated port may periodically resend the proposal BPDU if it doesn’t receive an agreement within the expected timeframe.
4. While waiting for the agreement, the port remains in the listening and learning states, continuing to receive and process BPDUs.
5. If the agreement is not received within a certain period, the switch may revert to traditional Spanning Tree Protocol (STP) behavior, resulting in a longer convergence time as the port transitions through the usual STP states (blocking, listening, learning, and finally forwarding).

The RSTP proposal and agreement mechanism is designed to rapidly transition ports to the forwarding state while maintaining loop-free topologies. The absence of an agreement BPDU halts the transition to forwarding to prevent potential network loops, or the operation falls back to STP.

Links

https://networklessons.com/spanning-tree/rapid-spanning-tree-rstp https://networklessons.com/spanning-tree/rapid-spanning-tree-configuration