First off, I feel quite dumb when it comes to the advanced possibilities that OpenWrt offers. I do know the basic stuff about networks like netmasks, broadcast ranges, routing tables and so on. However, I have only had to deal with my simple home network consisting of a single OpenWrt-router with Ethernet switches and wireless access points.
Now I would like to extend the installation by adding, among other things, an additional LTE-connection that is served by a second OpenWrt-router. For reasons of high-availability I would like to have that router connect to two different local networks (10.0./16 and 10.1./16). There does not have to be any routing between them but only further on to the WAN. The devices are fairly modern, i.e. less than five years old, and reasonably powerful. They are running OpenWrt 24.10.
The extra twist is that there is only one cable available for both network connections. I have a feeling that tagged VLAN might be the solution strategy and I have tried multiple times to read up about VLANs. But since I don't have any practical experience I do not even know if this is the right field to search for a solution. Can anybody please point me in the right direction and comment on how to go about this problem? Thanks in advance!
High availability is a rather complex thing, and is probably not actually satisfied by your proposal. Could you draw a network topology diagram?
Also, why are you using /16? while it will function just fine, this is considered unnecessary for the vast majority of situations and only serves to add complications in certain situations.
Yes, probably VLANs, but we need to see the proposed topology to ensure we are advising appropriately.
I don't understand your requirements regarding high availability either.
Does it really have to be high availability, or should an alternative Internet connection be available just in case the wired Internet connection fails?
Because then you don't need a second router...
VLANs are ideal for handling several independent networks via one cable. But all of your network devices need to be VLAN aware.
i.e. managed switches.
Take a look at onemarcfifty's videos on this topic.
It is explained there in a way that is very easy to understand.
https://www.youtube.com/@OneMarcFifty/videos
Many thanks for your comments @psherman and @fow0ryl ! As luck had it I did stumble over onemarcfifty's videos right after this posting here. I think that explains a whole lot but I have to study it more thoroughly and experiment with it.
You are right that there is more to my set-up than what I mentioned in order while keeping things short. I was not hoping for an overall solution but now that you have asked I want to give a brief description of the situation: I have some servers that need internet access under "all" circumstances and I want to eliminate the SPOF in the router/internet connection. I am about to use mwan3 on the main router for a failover from cable internet to LTE on the secondary router. AFAIK this should be its intended purpose.
However, if the main router itself is not reachable, e.g. due to power cut, then I want the servers to use the secondary router as gateway. The devices shall have two default routes with different metrics for the two gateway routers. In order to avoid LAN-loops they need to be placed in separate subnets. Then it works perfectly well. Here is where the mentioned restriction of one single cable comes in. If it was not for that then I would simply use a separate LAN-cable.
Please feel free to comment! I am happy to learn from others' expertise.
A 2 to 1 splitter, lots of different makes, for example:
