Out of personal interest I am working my way through the IEEE standards 802.1D-2004 and 802.1Q-2014. I am using the earlier document to clarify meaning in the later document, since the addition of MSTP and SPB adds a lot of complexity to the standard. Would anyone like to clarify the exact meaning of 802.1D-2004, Section 17.6 final para: "If the designated priority vector is better than the port priority vector, the Port will be the Designated Port for the attached LAN and the port priority vector will be updated."? There seems to be a circularity with Para 1: "The port priority vector is the spanning tree priority vector held for the port when the reception of BPDUs and any pending update of information has been completed." If the reception and pending updates have been completed, how can the vector be later updated? Just to be clear, I am not asking for clarification about the operation of RSTP. I am asking for clarification of the meaning of the algorithm expressed in those paragraphs.

  • "how can the vector be later updated?" The bridges regularly send BPDUs, and will update based on changes reflected in the BPDUs.
    – Ron Maupin
    Commented Aug 22, 2018 at 16:16
  • Thanks, that is true of the way RSTP works, I agree. But I don't see how it clarifies those two paragraphs in Section 17.6. According to the definitions: a "designated priority vector" contains a port identifier; the "port priority vector" is what you get after completing the update; but a designated priority vector replaces a port priority vector as the new port priority vector if it is superior. So in what sense is "the reception of BPDUs and any pending update of information" completed?
    – Anthony
    Commented Aug 22, 2018 at 16:37
  • The wording has been retained in the equivalent section of 802.1Q-2014, so I assume the authors intended those words precisely. In this case I think they must mean "where the reception of BPDUs and any pending updates of the port priority vector from the information in those BPDUs have been completed". Then the port priority vector is evaluated against the designated priority vector and replaced if inferior.
    – Anthony
    Commented Aug 23, 2018 at 17:57
  • OK, I think the answer to my question is in Section 17.27 Port Information state machine. A summary of this is that the received BPDU information is processed and updates the current port information. A separate update of the current information then occurs if the designated priority vector is better than the port priority vector.
    – Anthony
    Commented Aug 24, 2018 at 12:12
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin
    Commented Dec 25, 2018 at 9:20

1 Answer 1


Answer above, it is a two-step update process.

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