Network Architecture Redundancy Question

Hi,

I have a question about the ring network topology and redundancy. The network will be on a fiber optic media. I've been doing some reading and found out about switches that use a spanning tree protocol to help find a new path if a link was ever broken. My question is, when it comes to the physical construction (laying out the fibre cable) are you basically using two separate fiber cables (16 or whatever strands each cable) to build two rings (inner ring and outer ring) and if there is a break in one ring the spanning tree protocol will reroute to the other ring or use the other ring to heal the break? Of course this will require double the amount of fiber cable.

Or

Do you just lay one cable in a ring topology and if there's a break the rapid spanning tree routes the traffic to go the other direction therefore not needing the other ring? Can a ring topology transmit in both directions?

Also i'm just strictly talking about one break in the ring...if there are multiple breaks, its assumed the station that between the broken link can't be communicated to.

I'm designing a network architecture (its my first network design) where i'd like communications from two buildings (about 15 kms apart) but i'm picking up about 15 other stations in between, but i'd also like to keep costs down while keeping the network as reliable as possible.

The typical scalable architecture for multiple locations with redundancy is somewhere between a ring and mesh of routers, with switches dangling off. As the network gets bigger, you typically distinguish between "core", "distribution", and "access". You can achieve some redundancy with extra links between switches, but it probably isn't the end all solution you're thinking. I'll clarify:

Managed switches typically support multiple uplink ports between them - in your case using either single mode or multimode fiber. You can wire your switches together physically however you want (ring, double ring, mesh, etc). Keep in mind that only one path to each node will be used. (Rapid) Spanning Tree Protocol will effectively shut down unused ports, creating a logical "tree" that is "loop free". A double loop, versus a single loop could theoretically withstand losing many fiber strands, but in the case you described, both of those strands of fiber would likely be run together. Losing connections is a big deal - something that is actionable to you and should be a rare event.

In any event - keep in mind that you're using an Ethernet network. You don't need a ring like a Token Rink network. Routers deal with multiple paths at layer 3 (IP). When properly configured, more paths = more bandwidth + fault tolerance. Extra, redundant layer 2 switch connections means more ports will be in a "blocked" state at a given time. You have to be especially careful if the connections are different speeds or you could find your network choosing only the slower ones.

All that said - it's often not much more expensive to lay multiple strands of fiber in a bundle as opposed to a single one. Always a good idea for growth.