Use a single smart switch in a network loop?

(Forgive me, our network guys are only here once a month, so we are trying to do some research until then ...)

We have a pretty straightforward network here, but it is spread out over 4 buildings.
We want to make a network "loop", where each building is connected to the next, and then back around.  This is so that if one connection to a building fails, there is a another.

We believe we can do this with only one "smart" switch (located where the servers are), and unmanaged "dumb" switches in the other 3 buildings.  We are told that if the smart switch does "Spanning Tree Protocol", we can set up a loop in this manner.  That switch, we assume, will be a Cisco SG200 model.  

Here's our theory ... network loop diagram
In the example diagram, we would enable port 01 to be active, so all network traffic Building D would go through port 01, through the switches in B and C, then to D.
If the connection between A and B (in red) goes down, then the smart switch would bounce to port 02, and traffic to B would go through D and C.  Does that sound right?

Now, what if the connection between B and C dies ?  Would the Smart Switch (using STP) start allowing traffic through both port 01 and port 02, as the situation would require?

Any suggestions are appreciated.

BTW, Our backup servers are in Building B, in case Building A goes up in flames or something.
Rob RudloffIT Development SpecialistAsked:
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Craig BeckCommented:
You've pretty much got it.

The smart switch would block port 1 or port 2.  If the smart switch stops receiving its own BPDUs on the blocked port, that would indicate that the loop isn't in the network anymore (if a link between B and C, or C and D failed), therefore the blocked port would become unblocked and pass traffic.

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Yes, very basic and will work with a stp enabled switch.  The cisco SG200 has spanning tree protocol support and you just plug it in, and it will allow the redundant connection without the worry of bridge loops.
Rob RudloffIT Development SpecialistAuthor Commented:
-- thanks!
-- Looks  like we'll be giving this a try in a month.
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