Question for didactic purposes where I try to keep it IEEE bound and put Cisco off the discussion unless strictly necessary. I refer to 802.1D-1998 and the question is to understand, after clarifying what I write on this post, all the passages that brought Cisco to define PVST+ and then this with IEEE


Q: Correct me if I'm wrong but, as far as I understood IEEE says that STP-BPDUs must be untagged, but in a MST (Mono Spanning Tree) switch where a primary VLAN is defined, what happens to BPDUs on trunk where the PVID is not the same as the primary VLAN?


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1 Answer 1


IEEE BPDUs are always sent untagged. If you check out the current 802.1Q-2018 (free access after registration), the BPDU form is detailed in Clause 14 and MSTP itself in Clause 13.

Even with Multiple Spanning Tree Protocol (MSTP), a single untagged BPDU holds all information about all configured MST instances (max 64 MSTIs). MCIDs are used to make sure all bridges talk about the same VLANs/trees.

BPDUs are not forwarded, so it's easier to visualize them as not belonging to any VLAN. They're simply transmitted across a link without a tag.

  • Thanks @Zac67, I apologies for the obvious but I don't seem to see in the document a section for Clause 14 but only references to it. The same goes for Clause 13. Where are they? In which page? Sep 24, 2019 at 13:24
  • Not at all. Clause 13 starts on p. 467, Cl 14 on p. 554 - you need the full doc, not just the preview.
    – Zac67
    Sep 24, 2019 at 14:16

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