RSTP (802.1w) uses type 2, version 2 BPDUs so an RSTP bridge can communicate
802.1D on any shared link or with any switch running 802.1D. RSTP sends BPDUs
and populates the flag byte in a slightly different manner than in 802.1D:
- An RSTP bridge sends a BPDU with its current information every hello time
period (2 seconds by default), even if it does not receive any BPDUs from the
root bridge.
- Protocol information can be immediately aged on a port if hellos are not
received for three consecutive hello times or if the max_age timer expires.
- Because BPDUs are now used as a "keepalive" mechanism, three
consecutively missed BPDUs indicate lost connectivity between a bridge and its
neighboring root or designated bridge. This fast aging of the information
allows quick failure detection.
RSTP uses the flag byte of version 2 BPDU as shown in the figure:
- Bit 0 and 7 are used for topology change notification and acknowledgement
as they are in 802.1D.
- Bits 1 and 6 are used for the Proposal Agreement process
- Bits 2-5 encode the role and state of the port originating the BPDU