When a switch running Per-VLAN Spanning Tree Plus (PVST+) is added to a network with switches running Rapid PVST+, the network supports both protocols simultaneously due to the backward compatibility of Rapid PVST+ with PVST+. The existing switches will continue to operate using Rapid PVST+, while the newly added switch will operate using its configured protocol, which is PVST+. This ensures that there is no disruption in service and that spanning tree continues to prevent loops as expected.