A detailed description of the neighbor state changes follows. Each state change is invoked by an event (Section 10.2). This event may produce different effects, depending on the current state of the neighbor. For this reason, the state machine below is organized by current neighbor state and received event. Each entry in the state machine describes the resulting new neighbor state and the required set of additional actions.
When a neighbor's state changes, it may be necessary to rerun the Designated Router election algorithm. This is determined by whether the interface NeighborChange event is generated (see Section 9.2). Also, if the Interface is in DR state (the router is itself Designated Router), changes in neighbor state may cause a new network links advertisement to be originated (see Section 12.4).
When the neighbor state machine needs to invoke the interface state machine, it should be done as a scheduled task (see Section 4.4). This simplifies things, by ensuring that neither state machine will be executed recursively.
State(s): Down
Event: Start
New state: Attempt
Action: Send an Hello Packet to the neighbor (this neighbor
is always associated with a non-broadcast network)
and start the Inactivity Timer for the neighbor.
The timer's later firing would indicate that
communication with the neighbor was not attained.
State(s): Attempt
Event: HelloReceived
New state: Init
Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has now been heard from.
State(s): Down
Event: HelloReceived
New state: Init
Action: Start the Inactivity Timer for the neighbor. The
timer's later firing would indicate that the
neighbor is dead.
State(s): Init or greater
Event: HelloReceived
New state: No state change.
Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has again been heard from.
State(s): Init
Event: 2-WayReceived
New state: Depends upon action routine.
Action: Determine whether an adjacency should be established
with the neighbor (see Section 10.4). If not, the
new neighbor state is 2-Way.
Otherwise (an adjacency should be established) the
neighbor state transitions to ExStart. Upon
entering this state, the router increments the DD
sequence number for this neighbor. If this is the
first time that an adjacency has been attempted, the
DD sequence number should be assigned some unique
value (like the time of day clock). It then
declares itself master (sets the master/slave bit to
master), and starts sending Database Description
Packets, with the initialize (I), more (M) and
master (MS) bits set. This Database Description
Packet should be otherwise empty. This Database
Description Packet should be retransmitted at
intervals of RxmtInterval until the next state is
entered (see Section 10.8).
State(s): ExStart
Event: NegotiationDone
New state: Exchange
Action: The router must list the contents of its entire area
link state database in the neighbor Database summary
list. The area link state database consists of the
router links, network links and summary links
contained in the area structure, along with the AS
external links contained in the global structure.
AS external link advertisements are omitted from a
virtual neighbor's Database summary list. AS
external advertisements are omitted from the
Database summary list if the area has been
configured as a stub (see Section 3.6).
Advertisements whose age is equal to MaxAge are
instead added to the neighbor's Link state
retransmission list. A summary of the Database
summary list will be sent to the neighbor in
Database Description packets. Each Database
Description Packet has a DD sequence number, and is
explicitly acknowledged. Only one Database
Description Packet is allowed outstanding at any one
time. For more detail on the sending and receiving
of Database Description packets, see Sections 10.8
and 10.6.
State(s): Exchange
Event: ExchangeDone
New state: Depends upon action routine.
Action: If the neighbor Link state request list is empty,
the new neighbor state is Full. No other action is
required. This is an adjacency's final state.
Otherwise, the new neighbor state is Loading. Start
(or continue) sending Link State Request packets to
the neighbor (see Section 10.9). These are requests
for the neighbor's more recent advertisements (which
were discovered but not yet received in the Exchange
state). These advertisements are listed in the Link
state request list associated with the neighbor.
State(s): Loading
Event: Loading Done
New state: Full
Action: No action required. This is an adjacency's final
state.
State(s): 2-Way
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether an adjacency should be formed with
the neighboring router (see Section 10.4). If not,
the neighbor state remains at 2-Way. Otherwise,
transition the neighbor state to ExStart and perform
the actions associated with the above state machine
entry for state Init and event 2-WayReceived.
State(s): ExStart or greater
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether the neighboring router should
still be adjacent. If yes, there is no state change
and no further action is necessary.
Otherwise, the (possibly partially formed) adjacency
must be destroyed. The neighbor state transitions
to 2-Way. The Link state retransmission list,
Database summary list and Link state request list
are cleared of link state advertisements.
State(s): Exchange or greater
Event: SeqNumberMismatch
New state: ExStart
Action: The (possibly partially formed) adjacency is torn
down, and then an attempt is made at
reestablishment. The neighbor state first
transitions to ExStart. The Link state
retransmission list, Database summary list and Link
state request list are cleared of link state
advertisements. Then the router increments the DD
sequence number for this neighbor, declares itself
master (sets the master/slave bit to master), and
starts sending Database Description Packets, with
the initialize (I), more (M) and master (MS) bits
set. This Database Description Packet should be
otherwise empty (see Section 10.8).
State(s): Exchange or greater
Event: BadLSReq
New state: ExStart
Action: The action for event BadLSReq is exactly the same as
for the neighbor event SeqNumberMismatch. The
(possibly partially formed) adjacency is torn down,
and then an attempt is made at reestablishment. For
more information, see the neighbor state machine
entry that is invoked when event SeqNumberMismatch
is generated in state Exchange or greater.
State(s): Any state
Event: KillNbr
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements. Also, the Inactivity Timer is
disabled.
State(s): Any state
Event: LLDown
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements. Also, the Inactivity Timer is
disabled.
State(s): Any state
Event: InactivityTimer
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements.
State(s): 2-Way or greater
Event: 1-WayReceived
New state: Init
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements.
State(s): 2-Way or greater
Event: 2-WayReceived
New state: No state change.
Action: No action required.
State(s): Init
Event: 1-WayReceived
New state: No state change.
Action: No action required.