Hi
Portfast functionally has the same effectiveness whether you run PVST+, Rapid-PVST, or any other spanning tree protocols
Rapid-PVST+ uses the same BPDU format as the 802.1D and it is backward compatible. It is difficult to convert all the switches in the enterprise network at the same time to rapid-PVST+. Because of the backward compatibility, you can convert phase by phase. It is recommended to implement the changes in the scheduled maintenance window because the spanning tree reconfiguration disrupts the traffic flow. Spanning Tree UplinkFast and BackboneFast features are PVST+ features. These are disabled when you enable rapid-PVST+ because those features are built within rapid-PVST+. Therefore, during the migration you can remove those commands. The configuration of the features such as PortFast, BPDUguard, BPDUfilter, root guard, and loopguard are applicable in rapid-PVST+ mode also. The usage of these features are the same as in PVST+ mode. If you have already enabled these features in the PVST+ mode, the features remain active after the migration to rapid-PVST+ mode. In this example, migration occurs in this order:
http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b0670.shtml#rpvst
HTH
Reza
