A router MUST have a mechanism to allow routing software to determine whether a physical interface is available to send packets or not; on multiplexed interfaces where permanent virtual circuits are opened for limited sets of neighbors, the router must also be able to determine whether the virtual circuits are viable. A router SHOULD have a mechanism to allow routing software to judge the quality of a physical interface. A router MUST have a mechanism for informing the routing software when a physical interface becomes available or unavailable to send packets because of administrative action. A router MUST have a mechanism for informing the routing software when it detects a Link level interface has become available or unavailable, for any reason.
It is crucial that routers have workable mechanisms for determining that their network connections are functioning properly. Failure to detect link loss, or failure to take the proper actions when a problem is detected, can lead to black holes.
The mechanisms available for detecting problems with network connections vary considerably, depending on the Link Layer protocols in use and the interface hardware. The intent is to maximize the capability to detect failures within the Link-Layer constraints.