State changes can be effected by a number of events. These events are pictured as the labelled arcs in Figure 11. The label definitions are listed below. For a detailed explanation of the effect of these events on OSPF protocol operation, consult Section 9.3.
Lower-level protocols have indicated that the network interface is operational. This enables the interface to transition out of Down state. On virtual links, the interface operational indication is actually a result of the shortest path calculation (see Section 16.7).
The Wait Timer has fired, indicating the end of the waiting period that is required before electing a (Backup) Designated Router.
The router has detected the existence or non-existence of a Backup Designated Router for the network. This is done in one of two ways. First, an Hello Packet may be received from a neighbor claiming to be itself the Backup Designated Router. Alternatively, an Hello Packet may be received from a neighbor claiming to be itself the Designated Router, and indicating that there is no Backup Designated Router. In either case there must be bidirectional communication with the neighbor, i.e., the router must also appear in the neighbor's Hello Packet. This event signals an end to the Waiting state.
There has been a change in the set of bidirectional neighbors associated with the interface. The (Backup) Designated Router needs to be recalculated. The following neighbor changes lead to the NeighborChange event. For an explanation of neighbor states, see Section 10.1.
An indication has been received that the interface is now looped back to itself. This indication can be received either from network management or from the lower level protocols.
An indication has been received that the interface is no longer looped back. As with the LoopInd event, this indication can be received either from network management or from the lower level protocols.
Lower-level protocols indicate that this interface is no longer functional. No matter what the current interface state is, the new interface state will be Down.