RSTP and the Alternate (Root) Port
With STP, each nonroot switch places one port in the STP root port (RP) role. RSTP follows that same convention, with the same exact rules for choosing the RP. RSTP then takes another step, naming other possible RPs, identifying them as alternate ports.
To be an alternate port, that switch port must also be hearing a hello BPDU that declares the same switch to be the root switch. For instance, in Figure 11-10, SW1 is the root. SW3 will receive hello BPDUs on two ports: G0/1 and G0/2. Both hellos list SW1’s BID as the root switch, so whichever port is not the root port meets the criteria to be an alternate port. SW3 picks G0/1 as its root port in this case, and then makes G0/2 an alternate port.
Figure 11-10 ...
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