Spanning Tree Across multiple layer 2 switches

Hi there experts.    I will try to describe this the best I can ... this setup is actually tested and working but what I am not sure about is whether its recommended and normal.      Or that it might cause network issues now or in the future.     We have a third party product which essentially is another completely separate physical network from ours.    They have there own custom network switches.    We are trying to setup redundant links between our network and the third party.     The third party network is not manageable so no configuration is possible.    We were advised to just enable Spanning Tree on our switches and plug in 2 ports from the same VLAN and it would work.   Sure enough it does.    However from my very basic knowledge of networking this seems wrong.     It seems to be creating a loop?
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Don JohnstonInstructorCommented:
It's definitely not a preferred topology. But it's not the end of the world. ;-)

Here's the thing with unmanaged switches: They behave like hubs when it comes to spanning-tree.  In other words, STP BPDU's will flow through those switches unchanged.

So as far as your STP capable switches are concerned, the cable that goes from your switch, through the third party network, and then back to your switch, appear to be a single cable connecting two ports of your switch together.

Which means, that one of your two ports should be blocking.

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Zoldy2000Author Commented:
thank you for the response... what if the 2 ports on our side are on 2 different switches but same VLAN...?      Same scenario?
Don JohnstonInstructorCommented:
Yep.  Same scenario.  One of the ports on one of the switches should be blocking.
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