RSTP interoperates with legacy STP protocols; however, the rapid convergence benefits are lost when interacting with legacy STP bridges.
Each RSTP bridge contains a port protocol migration state machine to ensure that the ports in the STPD operate in the correct, configured mode. The state machine is a protocol entity within each bridge configured to run in 802.1w mode. For example, a compatibility issue occurs if you configure 802.1w mode and the bridge receives an 802.1D BPDU on a port. The receiving port starts the protocol migration timer and remains in 802.1D mode until the bridge stops receiving 802.1D BPDUs. Each time the bridge receives an 802.1D BPDU, the timer restarts. When the port migration timer expires, no more 802.1D BPDUs have been received, and the bridge returns to its configured setting, which is 802.1w mode.