State changes can be effected by a number of events. These events are shown in the labels of the arcs in Figures 12 and 13. The label definitions are as follows:
A Hello packet has been received from a neighbor.
This is an indication that Hello Packets should now be sent to the neighbor at intervals of HelloInterval seconds. This event is generated only for neighbors associated with non- broadcast networks.
Bidirectional communication has been realized between the two neighboring routers. This is indicated by this router seeing itself in the other's Hello packet.
The Master/Slave relationship has been negotiated, and DD sequence numbers have been exchanged. This signals the start of the sending/receiving of Database Description packets. For more information on the generation of this event, consult Section 10.8.
Both routers have successfully transmitted a full sequence of Database Description packets. Each router now knows what parts of its link state database are out of date. For more information on the generation of this event, consult Section 10.8.
A Link State Request has been received for a link state advertisement not contained in the database. This indicates an error in the Database Exchange process.
Link State Updates have been received for all out-of-date portions of the database. This is indicated by the Link state request list becoming empty after the Database Exchange process has completed.
A decision must be made (again) as to whether an adjacency should be established/maintained with the neighbor. This event will start some adjacencies forming, and destroy others.
The following events cause well developed neighbors to revert to lesser states. Unlike the above events, these events may occur when the neighbor conversation is in any of a number of states.
A Database Description packet has been received that either a) has an unexpected DD sequence number, b) unexpectedly has the Init bit set or c) has an Options field differing from the last Options field received in a Database Description packet. Any of these conditions indicate that some error has occurred during adjacency establishment.
An Hello packet has been received from the neighbor, in which this router is not mentioned. This indicates that communication with the neighbor is not bidirectional.
This is an indication that all communication with the neighbor is now impossible, forcing the neighbor to revert to Down state.
The inactivity Timer has fired. This means that no Hello packets have been seen recently from the neighbor. The neighbor reverts to Down state.
This is an indication from the lower level protocols that the neighbor is now unreachable. For example, on an X.25 network this could be indicated by an X.25 clear indication with appropriate cause and diagnostic fields. This event forces the neighbor into Down state.