The purpose of regions in MSTP

[Mitrixsen]

Hello, everyone.

I am studying MSTP and I was configuring regions. I would therefore like to verify whether my understanding of them is correct since I wasn't quite sure what was the point of them at first, but now I believe I understand.

When it comes to MSTP, a region is just a group of switches that share the same MSTP configuration, correct? It is defined by several parameters. I’ll mention one which are the instances and the VLANs mapped to them.

The moment we have different MSTP configuration, more regions end up being created. So if I have 10 switches and I define the following instances:

SW1-5
MSTI1 - VLANs 1-500
MSTI2 - VLANs 501-1000

SW6-10
MSTI1 - VLANs 1-20
MSTI2 - VLANs 21-30

This will create two regions, with each region having 5 switches that share the same configuration.

Now, to confirm, if these switches were in the same region, this could cause problems and complexity, right? I suppose it would be problematic if each instance was configured differently (had a different root bridge, port costs, etc) and then each switch would have to go ahead and compare those overlapping VLANs in the instances and apply the relevant configuration only to some of them, and not to mention that it would be inconsistent and harder to configure/maintain, and so on…

So is it right to say that to preserve clarity, configuration consistency, and ensure a loop-free topology, the switches are grouped into regions and potentially separated into multiple of them if their configuration differs?

David

[Martin L]

Having multiple regions would make lab more complex and potentially unstable as u would have multiple domains each including sub-domian. Inside of a region and as well as between 2 or more regions you must ensure a loop-free topology.   "When multiple regions connect together, every region needs to construct its own IST and all regions should build one common CIST spanning across the regions." If you have 9 switches, perhaps re-creating INE lab based on their  blog is a good idea.   

Furthermore, lets look at the last 4 major revisions of CCIE R&S and Enterprise (including personal Lab experiences with several minor revisions).  All of them, so far, included single-region switching domain running MSTP or RSTP.  you were ask to configure a single-region MSTP with several vlan instances.     

Regards, ML
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[MHM Cisco World]

https://www.youtube.com/watch?v=h8iuNNfiDW4

this video explain the IST and CST in MST 
take Look if you have any Q ask I will try my best to explain topic in video 

MHM

[Giuseppe Larosa]

Hello @Mitrixsen ,

MST requires a complete different approach for operations when compared to PVST+ or Rapid PVST.

With PVST+ or Rapid PVST we can make changes on the fly for example we can decide what device will be the root bridge for a new VLAN when we create it by using spanning-tree vlan x priority Y.

This just causes a recalculation for the involved VLAN.

With MST changes on the fly are not impactless, because when we change the STP instances to VLANs mapping we create MST regions.

However, MST can be provisioned in advance in network design phase.

the whole IEEE Vlan space 1 to 4094 can be partitioned in subsets and each subset can be associated to an MST instance. This can be done before the VLANs are actually created on the switches.

During operations when a new VLAN is needed , the new VLAN id is picked from the subset associated to the MST instance that have the desired topology i.e. the root bridge placement.

This aspect of MST was explained first  by Cisco expert Francois Tallet here in the forums.

Now, going back to your questions, MST is suitable for environments that use many VLANs hundreds or even more then 1,000 and/or for multi vendor networks being a standard.

Just looking at Cisco switches there are some low end switches that have a limitation on the number of STP instances that can run at the same time. This number can be 256 in some models. It has been even lower in older models.

MST regions provide an additional scalabilty when used wisely: the details of each region are kept within the region itself and the different regions at border links exchange only CST BPDUs and the resulting inter region common instance is named CIST and can also handle single non MST capable switches and multiple MST regions.

Each MST region elects a regional root and it has a master port to talk with outside world as explained here:

https://community.cisco.com/t5/switching/multi-region-mstp/m-p/1592572

>> a region has to have exactly one forwarding port (the so-called Master port) connecting the entire region to the outside world. The role of this port is very similar to a root port role on a normal switch. The bridge with the Master port is therefore very important to the entire region because it carries all inter-region traffic. So a boundary switch that is closest to the global root is elected the regional root, and its root port is elected as the Master port

Multiple MST regions can be used in a service provider environment, they are less used in an enterprise.

Edit:

As a final note the MST configuration can be propagated by VTP version 3 using a separate database from a primary VTP server to all switches in the VTP domain/ MST region (single MST region )

Hope to help

Giuseppe