Rapid Spanning-Tree (RSTP)


I didnt understand some points on RSTP Negotiation process,

  1. What are they "sync"in? If both sides are in forwarding it means synced? If one side is failed and is non designated, does it mean it is unsynced for RSTP point of view?
  2. In which situations negotiation process is beginning? The moment we plug a switch to another switch for example, in that case they are starting to use negotiation process always, right?
  3. what is The success criteria for sending Agreement BPDU to root switch ? Being in forwarding state of ports in SW2?

Hello Görgen

The term “sync” here is referring the negotiation process that is taking place between RSTP switches. It doesn’t refer to a state of being “in sync” when both sides are in a particular stage. The negotiation itself is called “sync” or “synchronizing”.

During this convergence process, the switches actually send each other what are known as “sync” message to prepare them for upcoming changes in the network topology.

The beginning of the negotiation process is indeed when the switches are first plugged in.

The successful negotiation results in a stable state or topology. After the sync, proposal, and agreement messages, when the switches and their ports obtain a particular role (forwarding or blocked) and it remains stable, the result is a successful RSTP reconvergence.

For more information, take a look at these NetworkLessons notes:

I hope this has been helpful!


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Quick question - on the topology with the two half-duplex, shared links to the hub from SW2 and from SW3, if there were only one half-duplex link on each side of the hub, what would the port roles be on SW2 and on SW3? Would it be designated on SW2 and alternate on SW3? Or would it be backup on SW2 and alternate on SW3?

Hello Matt

I don’t see a topology using a hub or links functioning in half-duplex mode in this lesson. Just to clarify, Rene stated in the lesson that IF you have a hub connected to the network, then the switches that connect to that hub will consider their ports connected to the hub as shared. If they are shared, they cannot and will not enter the forwarding state immediately.

Other than that, the functionality of RSTP remains unchanged.

Just a note, the term “hub” used here refers to the network device, not the topology location. In other words, we’re not talking about the hub of a hub and spoke topology.

I hope this has been helpful!