802.1Q Tagged BPDU? 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? Alex

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.