Understanding How STP Topology Changes Propagate and Converge

Hello,

Asking a "simple" network question isn't ever easy is it? Thanks.

I'm building a network for a control and monitoring system that currently is made up of 14 locations anywhere from 1 km to 10 km via the single mode fiber we are installing. Currently, I'm using a layer 2 network over some 900MHz licensed bandwidth radios and transitioning to this new fiber network. These radios are layer 2+ switches that have static routing functions, firewall rules, and STP. Everything else I am looking at will have RSTP. The radios eventually will be a backup medium for this network and will play double duty as static routing between each remote locations LAN and the core fiber network described below. Currently, they act as a static router between each remote LAN and the Radio Network.

I just got finished reading u/VA_Network_Nerd 's post on STP from a year or so which was awesome. Reading how he looks at the priority setting of each switch helped me understand a lot. Unfortunately for me I have 6 Core switches that are connected by fiber in 2 bad "square" rings using his terminology. That's how it was run because of the geography of all of our remote sites.

It's much like the '8' character on a old 7-segment display:

EDIT: sorry don't know why the spaces are removed between my '|' characters. This should have Root Connected to SW-A and SW-A Connected to SW-B. That isn't showing up like that after I posted it.

SW-E ---- Root

| |

SW-D ---- SW-A

| |

SW-C ---- SW-B

​

QUESTION: Is the following how STP would work to fix a lost connection on my core network?

Let's say the switch B's (SW-B) root port goes through fiber to SW-A. Then up to the root switch from there. If the link between SW-B and SW-A is cut, SW-A would notify the root switch of the topology change and the SW-B would be lost as it no longer has a root port. Once the Root Switch knows of the change a broadcast goes out and STP figures out how best to re-direct traffic.

In my system SW-B would change its root port to communicate with SW-C then go up through SW-D, SW-A, and on to the Root. I'm assuming that after any topology change STP propagates through the entire core network and figures out the root ports and the backup ports for me automatically based on the STP Priority setting of each switch.

A remote site connected to SW-B on this core network is connected as follows:

SW-B (SFP) <--5km of fiber--> (SFP) Local SW Layer 2 (RJ45) <--CAT6--> (RJ45 on Core subnet) layer 2+ radio switch (RJ45 on Local Subnet) <--CAT6--> local layer 2 switch <--CAT6--> End Device

I am using the radio switch as a static router between subnets so I am guessing that the previous topology change would stop at the radio switch's RJ45 port as it is the last device passing layer 2 STP messages on the core network subnet. At least assuming my firewall rules in the layer 2+ radio switch block the Spanning tree traffic which, as configured now, I believe they do.

QUESTION: Does that mean the network won't heal until the STP messages figure out all new MAC tables from every end device back to the core switch?

QUESTION: If the core switches are using RSTP and the end points use STP, all devices in my network would really revert to STP in this outage example, correct? In that case having RSTP in the core switches really doesn't offer me any benefit it seems. From the core network root switch to each of the end devices on the core fiber network has to be re-figured out over the entire network after each topology change. It doesn't just figure out SW-A to SW-B doesn't work so change SW-B to communicate with SW-C and it is all done? Which is why STP takes so long to converge.