management

The endpoint count is larger than the node count and that surprises everyone building their first cluster. What has to be reachable, why this network must survive when every other one is down, and the security posture it needs because it can power-cycle the entire fleet.

Most architecture diagrams show one network and a cluster has four. Each has a different traffic pattern, a different failure consequence and a different port count, and the one that is under-counted every time is the one you need when everything else is broken.

Three networks, not one, and the mistake that hurts most is putting two of them on the same wires. The port and switch counts for each, the cable plan, the split of the bill between switches and optics, and the two decisions that are hard to change once the cluster is racked.