I know that MST using regions so that it doesn't have to run multiple instances of spanning tree. However on a trunk are the BPDUs no longer tagged (dot1q) and sent to the neighbor device. i.e are they only sent untagged ?



2 Answers 2


The short answer is that MSTP actually uses a special instance in addition to the user defined ones to handle all MSTP control traffic, called the Internal Spanning Tree instance (IST0).

From: http://blog.ine.com/2010/02/22/understanding-mstp/

The IEEE 802.1s implementation does not send BDPUs for every active STP instance separately, nor does it encapsulate VLAN numbers list configuration messages. Instead, a special STP instance number 0 called Internal Spanning Tree (IST aka MSTI0, Multiple Spanning Tree Instance 0) is designated to carry all STP-related information. The BPDUs for IST contain all standard RSTP-style information for the IST itself, as well as carry additional informational fields. Among those fields are configuration name, revision number and a hash value calculated over VLANs to MSTI mapping table contents. Using just this condensed information switches may detect mis-configuration in VLAN mappings by comparing the hash value received from the peer with the local value.

You should definitely check out the rest of that white paper, it's a great resource.

  • 5
    On some systems, it's called "CIST" - Common Instance Spanning Tree.
    – Ricky
    Jun 22, 2015 at 20:08
  • Sure is, thanks for the addition @RickyBeam. Jun 22, 2015 at 20:10

BPDUs are only tagged in Cisco's PVST/PVST+ and other vendor-specific variants.

All BPDUs in IEEE standards-based STP variants eg: 802.1d, 802.1w, 802.1s (STP, RSTP, MSTP) are sent without any 802.1Q header.

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