Hello itech, ciao Giuseppe,
Please allow me to join.
Indeed, MSTP BPDUs are always untagged - or better said, they are not sent in any specific VLAN. You may see that MST BPDUs are tagged - but if they are, it is because they are priority-marked and the VLAN ID in that case should be set to 0. It does not matter what is the native VLAN of the trunk, and it does not matter what VLANs or MSTI instances are created and how they are mapped to each other.
MSTI instance 1 vlan 9
MSTI instance 0 vlan 4 5
Switch1 and Switch2 is trunk and only vlan 9 is allowed -> does MSTP BPDU still go through this trunk untagged ?
Yes, it will still be untagged. An MST BPDU can be split into two parts. The first part has the same structure as RSTP BPDUs, and carries the parameters for the Common Spanning Tree (CST - the abstraction of the entire spanning tree of the network that treats each region as a single switch, and only takes the inter-region links into account). Then comes an MST extension part that starts with the MST configuration parameters (region name, revision number, MD5 digest of the VLAN-to-instance mapping table), and then it carries information about internal instances in the region - first about the Internal Spanning Tree of the region (IST or MSTI 0) which will always be present, and then about any additional instances if there are any. Information about instances is always carried inside the BPDUs, and is not deduced from an outer VLAN tag - there should be none.
Check some packet captures, for example, from this page:
https://packetlife.net/captures/protocol/stp/
Can i check what is the root switch for VLAN 4 and 5 ?
Giuseppe said very correctly that you can display the information about instances using show spanning-tree mst instance-id syntax. On Cisco switches, you can continue using the show spanning-tree vlan vlan-id syntax as well - the operating system is smart enough to find out which MST instance is the VLAN mapped into, and will automatically display the MST instance information instead.
Best regards,
Peter
