I have the feeling the more I read about MSTP, the more confused I get. So here we are in order to make things clear once and for all.

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/configuration/guide/cli_rel_4_0_1a/CLIConfigurationGuide/MST.html says:

A CIST is a collection of the ISTs in each MST region. The CIST is the same as an IST inside an MST region, and the same as a CST outside an MST region.

Later in the document it says:

An MST region looks like a single switch to the CIST.

Which contradicts the sentence above, AFAIAC.

Now to the questions:

  1. Now what exactly are the CST, the IST, and the CIST? Can someone draw me a simple MSTP topology with CST in red, IST in yellow, and CIST in green, so as to have a final reference without any ambiguous texts to be interpreted? :)
  2. Which of them is relevant considering the actual protocol, i.e. the BPDUs? I.e. What happens when an ISL inside a region drops? What happens when an ISL between regions drops? Which parts of the network will recompute their topology and why?

enter image description here

  • 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 7, 2017 at 21:45

3 Answers 3


Now what exactly are the CST, the IST, and the CIST? Can someone draw me a simple MSTP topology with CST in red, IST in yellow, and CIST in green, so as to have a final reference without any ambiguous texts to be interpreted? :)

I like to visualise them like this:

MSTP Instances

  • Wow you even used the colors I asked for :D So IYO there would be no links having multiple colors? Also in Msti1 wouldn't the regional root be the one going to switch A? Also we still have to find out what " The CIST is the same as an IST inside an MST region, and the same as a CST outside an MST region." means...
    – Marki
    Mar 18, 2016 at 6:53
  • Technically yes - the CIST would extend out to the ISTs, however these trees will already be built, so the topology of each would still remain the same - in other words, extend the green out across the yellow. As for the regional root - not necessarily. It is possible to set a CIST bridge priority as well as a IST Bridge Priority, to create the topology above, though in hindsight I probably should have highlighted this a bit better. As for the Cisco document terminology - my take is if you had an MST region with only one MSTI, then the topology of the CIST would be identical to the IST. Mar 18, 2016 at 22:26
  • The last part of that sentence in the docs doesn't read very well to me, but I suspect they are trying to convey that the CIST topology within the MST Region is hidden from the CST. Mar 18, 2016 at 22:29
  • I think we need to be careful when we talk about "only one". There is always at least one MSTI in every region which is the IST (MSTI0). So the CIST would then be comprised of all (non-blocking) links between or outside of regions (CST) and the non-blocking links of MSTI0 of every region. That would make sense. Everyone seems to mean something slightly different when you read about this stuff. So everyone either has it correctly figured out for themselves or they are just lucky what they are doing works.
    – Marki
    Mar 19, 2016 at 0:56
  • You may find it easier to skip Cisco's documentation then and head straight for the definitive source: standards.ieee.org/getieee802/download/802.1Q-2005.pdf Mar 19, 2016 at 5:02

You jumped ahead to configuring MST instead of reading the Cisco documentation describing MST, Understanding Multiple Spanning Tree Protocol (802.1s). This document has multiple, colored diagrams, and a pretty good explanation of terms.

The IEEE 802.1s committee adopted a much easier and simpler approach that introduced MST regions. Think of a region as the equivalent of Border Gateway Protocol (BGP) Autonomous Systems, which is a group of switches placed under a common administration.


In order to clearly understand the role of the IST instance, remember that MST originates from the IEEE. Therefore, MST must be able to interact with 802.1q-based networks, because 802.1q is another IEEE standard. For 802.1q, a bridged network only implements a single spanning tree (CST). The IST instance is simply an RSTP instance that extends the CST inside the MST region.

The IST instance receives and sends BPDUs to the CST. The IST can represent the entire MST region as a CST virtual bridge to the outside world.

It all makes a lot more sense with the diagrams.

Also see STP and MST:

IST, CIST, and CST Overview

Unlike other spanning tree protocols, in which all the spanning tree instances are independent, MST establishes and maintains IST, CIST, and CST spanning trees:

  • An IST is the spanning tree that runs in an MST region.

    Within each MST region, MST maintains multiple spanning tree instances. Instance 0 is a special instance for a region, known as the IST. All other MST instances are numbered from 1 to 4094.

    The IST is the only spanning tree instance that sends and receives BPDUs. All of the other spanning tree instance information is contained in MSTP records (M-records), which are encapsulated within MST BPDUs. Because the MST BPDU carries information for all instances, the number of BPDUs that need to be processed to support multiple spanning tree instances is significantly reduced.

    All MST instances within the same region share the same protocol timers, but each MST instance has its own topology parameters, such as root bridge ID, root path cost, and so forth. By default, all VLANs are assigned to the IST.

    An MST instance is local to the region; for example, MST instance 1 in region A is independent of MST instance 1 in region B, even if regions A and B are interconnected.

  • A CIST is a collection of the ISTs in each MST region.
  • The CST interconnects the MST regions and single spanning trees.

The spanning tree computed in a region appears as a subtree in the CST that encompasses the entire switched domain. The CIST is formed by the spanning tree algorithm running among switches that support the 802.1w, 802.1s, and 802.1D standards. The CIST inside an MST region is the same as the CST outside a region.

For more information, see the "Spanning Tree Operation Within an MST Region" section and the "Spanning Tree Operations Between MST Regions" section.

enter image description here

Only the CST instance sends and receives BPDUs, and MST instances add their spanning tree information into the BPDUs to interact with neighboring switches and compute the final spanning tree topology. Because of this, the spanning tree parameters related to BPDU transmission (for example, hello time, forward time, max-age, and max-hops) are configured only on the CST instance but affect all MST instances. Parameters related to the spanning tree topology (for example, switch priority, port VLAN cost, and port VLAN priority) can be configured on both the CST instance and the MST instance.

MST switches use Version 3 BPDUs or 802.1D STP BPDUs to communicate with 802.1D switches. MST switches use MST BPDUs to communicate with MST switches.

IEEE 802.1s Terminology

Some MST naming conventions used in the prestandard implementation have been changed to include identification of some internal and regional parameters. These parameters are used only within an MST region, compared to external parameters that are used throughout the whole network. Because the CIST is the only spanning tree instance that spans the whole network, only the CIST parameters require the external qualifiers and not the internal or regional qualifiers.

  • The CIST root is the root bridge for the CIST, which is the unique instance that spans the whole network.
  • The CIST external root path cost is the cost to the CIST root. This cost is left unchanged within an MST region. Remember that an MST region looks like a single switch to the CIST. The CIST external root path cost is the root path cost calculated between these virtual switches and switches that do not belong to any region.
  • The CIST regional root was called the IST master in the prestandard implementation. If the CIST root is in the region, the CIST regional root is the CIST root. Otherwise, the CIST regional root is the closest switch to the CIST root in the region. The CIST regional root acts as a root bridge for the IST.
  • The CIST internal root path cost is the cost to the CIST regional root in a region. This cost is only relevant to the IST, instance 0.
  • Can you tell me which part of the Cisco documents give an exact answer to any one of my questions? :-) I still don't get the sentence "The CIST inside an MST region is the same as the CST outside a region." I have added a diagram to my question.
    – Marki
    Mar 16, 2016 at 12:56

I've given this a lot of thought and I am attaching a technical drawing of my own. Especially the CIST and CST part in BenjaminDale's drawing didn't really make sense to me: why would the CIST be linking the instances? I currently see it the following way:

Spanning Tree Topology

Let's discuss.

  • Well we're in the process of establishing an answer here, are we not. :) I know posting an answer myself is somewhat unusual and I don't want to invalidate or seem unthankful of the works of the others. However I don't think I could've posted another diagram as a comment to BenjaminDale's answer. Here's the chat room.
    – Marki
    Apr 1, 2016 at 21:45
  • You are perfectly fine posting your own answer and accepting it (you are even encouraged to do this), but this is not a discussion forum. If you wish to discuss things, then the proper place is on Network Engineering Chat.
    – Ron Maupin
    Apr 1, 2016 at 21:47

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