Multiple Spanning Tree (MST)

[lagapidis (Lazarus Agapidis)]

Hello David

Your explanations and your process are excellent, thank you for sharing in such detail.

I stand corrected in my assumption in my previous post. The rule that is stated in Cisco documentation is the following:

If the CIST root is outside the region, the protocol selects one of the MST switches at the boundary of the region as the CIST regional root.

So in your case, when you connected the two regions, since the CIST root (SW5) was outside of the blue region’s IST, the blue region had to recalculate its own regional root bridge to be on the boundary, which is why SW2 was chosen.

Now how is the correct new regional root chosen? You are correct in your statement:

@davidilles

The regional root is the switch that is the closest to the neighboring region in terms of external path cost. The regional root for the blue region will be SW2 because it is the closest to the green region. SW2 and SW3 actually tie here, but SW2 wins because it has the lower STP priority.

Actually, to refine your statement just a bit, the IEEE 802.1Q-2005 states specifically that:

…within each MST region, the IST root should be the switch with the lowest path cost to the CST root. This ensures that traffic within the region takes the most efficient path to the CST root, minimizing the overall network path cost.

Thanks so much for your thoroughness and clarity!

I hope this has been helpful!

Laz