Receiving Bridge Behavior
The receiving bridge must decide whether or not to accept a proposal from a port.
Upon receiving a proposal for a root port, the receiving bridge:
Processes the BPDU and computes the new STP topology.
Synchronizes all of the designated ports if the receiving port is the root port of the new topology.
Puts all unsynced, designated ports into the blocking state.
Sends down further “propose” messages.
Sends back an “agree” message through the root port.
If the receiving bridge receives a proposal for a designated port, the bridge replies with its own BPDU. If the proposal is for an alternate or backup port, the bridge keeps silent.