Spanning Tree Protocol: RSTP BPDU Format

In RSTP there is only one BPDU used for building loop free topology and topology change notifications compared to 802.1d where there were two types of BPDUs…Hello BPDUs and TCN BPDUs.

802.1d only used 2 of the 8 bits in the Flags field of the BPDU to acknowledge and propogate topology changes, 802.1w uses the remaining 6 bits to encode the following additional information:

  • Proposal bit
  • Port Role bits
  • Learning bit
  • Forwarding bit
  • Agreement bit

TCA bit is not used in RSTP this allows the BPDU in RSTP to carry information about the originating port’s role and state allowing for RSTP’s dispute mechanism protecting against unidirectional link problems.

In 802.1d the Root switch was the only switch that could send configuration BPDUs to all other switches, and if a network event occurred switches would need to wait the MaxAge-MessagAge timeout before the original superior BPDU received by the now disconnected root switch is expired.

RSTP allows each switch to originate their own BPDUs with the contents based on the switches Root Port.  This allows the switch to Age out BPDUs much sooner, three missing Hellos in a row cause a port to age out the stored BPDU and the MessagAge field no longer has any influence on the BPDU expiration.



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