Why a switch loop refuses to start
netplex.'s unmanaged Switch is deliberately simple. It is a passive layer-2 switch with no spanning tree running on it. (netplex. never sends a packet of its own, and spanning tree would do exactly that.) So switches can not be wired in a loop, the way you could on real hardware that runs RSTP (a loop-blocking protocol). A loop would flood the host with broadcast traffic. netplex. catches this at design time, before you ever hit Start.
Three switches wired in a triangle - a textbook L2 loop. On real hardware, RSTP would block one port automatically; netplex. switches never run STP at all (see "How links work"), so this shape is refused instead of silently storming.
Click Start. The request is accepted (starting happens in the background), but the loop check runs as the lab comes up and rejects it moments later with a clear message - which link to remove - over the same live connection the rest of the UI uses for status.
Remove the redundant link the message named and Start works normally - same three switches, just no cycle between them.
This is the same zero-injection principle behind the rest of the fabric (see "How links work" and "The unmanaged switch") - netplex. never adds hidden protocol traffic to make a bad topology limp along, it tells you exactly what to fix instead.