Multiple spanning tree terminology (CST/IST/CIST) and exact behavior

spanning tree

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?

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Best Answer

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

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