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I have four switches node arranged in a ring topology; I am using RSTP or STP.

I learned the classic three switches ring topology. Is the four switches topology using the same method as three switches to decide the port state? If so, at what step and what is the different between RSTP and STP? How can I know what state the switch port is in besides testing it?

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  • show spanning-tree summary | i mode and show spanning-tree interface – This Jul 23 '13 at 14:30
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STP does not operate differently whether the network has three, four, or a dozen switches. Each switch applies the same logic locally, and don't even actually know how many switches exist within the network.

Legacy and rapid STP behave differently (you'll need to research the differences) but both operate on the same basic principle: calculate the best path to the root bridge and disable all redundant paths. If you know the topology of and the costs associated with each link in the network, it's entirely possible to calculate the best path on paper alone.

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Assuming that all link speeds are the same, things become hop count. If you explicitly set a root bridge the switch 'across' from it in the ring will have the blocked port, it will be the port with the higher MAC address.

There is no real difference in blocked port selection between rapid and 'classic' STP

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