8

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 ?

Thanks,

  • 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 could provide and accept your own answer. – Ron Maupin Aug 11 '17 at 17:57
6

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 Beam Jun 22 '15 at 20:08
  • Sure is, thanks for the addition @RickyBeam. – Jordan Head Jun 22 '15 at 20:10
1

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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.