Multi-region MSTP

[droeun141]

I am confused about single vs mult-region MST.  I know that in single region, the IST root is elected by the lowest bridge priority.  But in multi-region, there is also an additional root bridge election (regional root) with lowest cost path to the CST root.  The regional root also becomes the IST root for each given region, regardless of bridge priority.  What is the purpose behind this? why does each region need a regional root and why can't they just function using the IST root instead?

Thank you for any help in understanding.

[Peter Paluch]

Hello,

What is the purpose behind this? why does each region need a regional 
root and why can't they just function using the IST root instead?

I see two reasons for the need of the regional root:

• A region tries to appear as a single switch to the outside world. Thus, when the region talks to another region, it should present a single BID. This is accomplished by using the BID of the regional root as the "Sender BID" of a BPDU talking to the outside world. Please note that this has not always been the case - older implementations of MSTP did not care to use the regional root's BID as a "Sender BID" field. The most recent version of 802.1Q mandates this, however.
• More importantly, 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. From this, it is logical to make this switch a regional root and derive the regional internal IST topology with respect to this switch. In very layman terms, the root switch is the "highest" switch in the topology, and in a region, the regional root is elected as the "highest" switch in the region, and the remaining internal active topology of the IST is derived with this regional root as a reference point.

Please feel free to ask further!

Best regards,

Peter

[droeun141]

Are regional roots elected when connecting to standard STP/PVST+ switches too?

[Peter Paluch · Accepted solution]

Hello,

Are regional roots elected when connecting to standard STP/PVST+ switches too?

Yes, they are. A boundary switch does not make a difference whether the neighboring switch is in a different MSTP region or whether it is a pure STP switch. All it knows is that it is a boundary switch, and it uses only the IST to talk to the outside world.

Best regards,

Peter

[droeun141]

Ahh it makes sense now! At first I thought regional ports were elected only when talking to other MST regions.  But if the same is true for anything at the boundry then it makes sense now.  Thanks Peter!