Connecting links represent a service, for example an interface, that connects two devices. They are drawn as lines from one device to another. If two devices have mutual links, the single line can consist of more than one color (if one object is up and the other is down). The center-point of the line back to the up object is green, while the other half of the line going to the down object is red. In essence, the color of the line represents the state of the service on the host that the color touches.
If the red part of the line touches "System A" and the green part of the line touches "System B", then we know that some service on "System A" is the problem.