RSTP proposal-agreement process
When implementing RSTP as the Spanning Tree Protocol (STP) in a Layer 2 topology, the initial negotiation process that RSTP switches go through, or the proposal/agreement process, takes place like so:
Lets say we have SW1 and SW2 connected to each other, and SW1 will become the root bridge based on its configuration. As soon as a new link comes up between SW1 and SW2, both ports on either end of the link go into Designated Discarding state. Designated Discarding is the default state in RSTP as soon as a port comes up.
Every Designated Discarding port sends BPDUs with the proposal bit set to 1. This bit is set to indicate to the other switch that the initial negotiation is to take place. That means that both SW1 and SW2 will send these BPDUs. The BPDU being sent from SW1 to SW2 is the only relevant exchange since SW1 will be the root bridge. Any BPDU sent by SW2 to SW1 will simply be ignored.