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:
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Processes the BPDU and computes the new STP topology.
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Synchronizes all of the designated ports if the receiving port is the root port of the new topology.
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Puts all unsynced, designated ports into the blocking state.
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Sends down further “propose” messages.
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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.