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The

K3S_URL

essentially means "look at this server here and add me as a member to it's cluster" when you install k3s on a node it creates an etcd datastore with a single node 'cluster'. You can add other nodes that are on their own island as single nodes by syncing all of their databases together. You designate one of the nodes that everyone should register with and sync their databases with. Once they are all sync'd they have the same information and all can become a new registration endpoint for your cluster. You are assimilating the nodes to the borg (quite literally) and the

K3S_URL

is just a way for you to point to a master that has a registration endpoint (a master node).

Thanks again, this was illuminating! 🙂