RSTP uses type 2, version 2 BPDUs. The original 802.1D STP uses type 0, version 0 BPDUs. However, a switch running RSTP can communicate directly with a switch running the original 802.1D STP. RSTP sends BPDUs and populates the flag byte in a slightly different manner than in the original 802.1D:
- Protocol information can be immediately aged on a port if Hello packets are not received for three consecutive Hello times, six seconds by default, or if the max age timer expires.
- Because BPDUs are used as a keepalive mechanism, three consecutively missed BPDUs indicate lost connectivity between a bridge and its neighboring root or designated bridge. The fast aging of the information allows failures to be detected quickly.
Note: Like STP, an RSTP switch sends a BPDU with its current information every Hello time period (two seconds, by default), even if the RSTP bridge does not receive any BPDUs from the root bridge.
As shown in the figure, RSTP uses the flag byte of version 2 BPDU:
- Bits 0 and 7 are used for topology change and acknowledgment as they are in the original 802.1D.
- Bits 1 and 6 are used for the Proposal Agreement process (used for rapid convergence).
- Bits from 2 to 5 encode the role and state of the port.
- Bits 4 and 5 are used to encode the port role using a 2-bit code.