Non [R]STP switches in LAN ring topology

Question

I am revamping a simple network, which is based on cascaded, standard, unmanaged PoE switches. I am also adding a redundant loop closing wire, from the last switch to the first one, which at the moment, due to unavailability managed and [R]STP switches, is left open.

I am planning to use [R]STP or similar technologies but I have a crucial question about: Can I put a single STP switch only on the loop closing point and avoid changing the remaining switches in the middle (which would cost me a lot of money)?

For example, taking a switch like the Ubiquiti Toughswitch series (which would be handy thanks to its ability to power also their Airmax products), would it be possible to close the loop on two of its ports and enabling the spanning tree protocol to avoid creating a loop but keeping the ability to self recover the network in the failure event of one of the cascaded switches?

Thanks in advance for all the suggestions.

Answer 1

If you plan to create a ring of switches, all those switches need to support RSTP or MSTP.

That is, unless all the unmanaged switches completely ignore IEEE 802.1D and actually forward the RSTP BPDUs - which is very ugly but quite common. (That way, the RSTP switch sees its own BPDUs from one ring port coming in through the other one, and blocks the one with the higher interface number.)

You'd need to test that however, and if there's only a single unmanaged switch not forwarding the BPDUs, the loop goes undetected, a broadcast storm starts and takes down the network.

Beware of future changes (switch replacement or expansion) breaking this delicate setup. I'd seriously recommend using decent switches for a redundant setup. Also, with managed switches you might want to change the ring to a tree which provides better performance.