I am attempting to gain a better understanding of how STP converges when certain state changes occur that affect the STP topology. In this scenario (see below) Switch A (SwA) is the root switch with Switch B (SwB) and (SwC) connected by a single link both to SwA and to one another (so a mesh topology). In this case, the root ports for both SwB and SwC or those ports directly connected to SwA. Also in this case, SwB wins the designate port election based on the fact that it has the lower BID. Thus, it configures its port on the link that connects to SwC as a DP in a forwarding state with SwC configuring its corresponding port in state blocking.
My question is this: What exactly happens if SwB initial root fails (see below) and it now needs to select a new root? I understand what occurs in SwB's case. It will go through the STP process again and ultimately settle on using the port currently connected to SwC as its new root (formerly a designated port). What I don't understand it what happens on SwC since it has configured the port on itself that connects to SwB in a blocking state. My CCNA books states that a port in a blocking state receives Hello BPDUs, but discards them and does not process them. The book also states that a port in a blocking state will not send out Hello BPDUs. How, therefore, does SwC know that it now needs to configure this port as a designated port since, on the corresponding side, SwB will be using this link as its root? Would it be because it would cease receiving Hello BPDUs on this port and, after the expiration of the Max Age Timer, begin the process of changing the STP topology?
