Spanning Tree Protocol: RSTP Proposal/Agreement Process

Topology events can occur when a new switch is plugged into the existing switching infrastructure.  RSTP uses a Proposal/Agreement process on point-to-point links to rapidly put the port into a forwarding state without causing a disruption in services or creating a loop.

The Proposal signifies the willingness of a port to become Designated Forwarding….while the Agreement stands for allowing the port to begin forwarding immediately.

When a new point-to-point link is added between two switches, both ports come up as Designated Discarding, the default role.  Ports in the Discarding or Learning state send BPDUs with the Proposal bit set, both switches will do this assuming they have the right to be Designated.

If the BPDU received on the designated discarding port is determined to be the superior BPDU its role will change from Designated to Root discarding.  Port roles will also be updated in the process.

If a switch receives a BPDU with a proposal bit set on its Root port, it places all non-edge ports into a discarding state…this operation is called Sync.  A switch in Sync state is isolated from the network, preventing any loops from passing through it.

Once it is syncd the switch moves the new Root port into a Forwarding state and informs the upstream switch that it is now allowed to move its designated discarding or learning port to a forwarding state.  This is done by the switch sending a BPDU with the Agreement bit set through its Root Port after performing the Sync.

After receiving the Agreement the upstream switch will move the port into a forwarding state completing the Proposal/Agreement Process

As a result all designated non-edge ports were put into a discarding state, at this point they all begin sending BPDUs with the Proposal bit set and begin the Proposal/Agreement process downstream to the other switches.



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