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, both the RP and the alternate port must receive Hellos that identify the same root switch. For instance, in Figure B-1, SW1 is the root. SW3 will receive hello BPDUs on two ports: G0/1 and G0/2. Both hellos list SW1’s bridge ID (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 B-1
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