Implementing the Rapid Spanning Tree Protocol – RSTP
Examining the RSTP BPDU

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