notes; OSPF Finite State Machine

- An OSPF router transitions a neighbor through several states before the neighbor is considered fully adjacent:

1.  Down - The initial state of a neighbor conversation indicates that no hellos have been heard from
the neighbor in the last router dead-interval.  If a neighbor transitions to the down state, the link state retransmission, database summary, and link state request lists are cleared.

2.  Attempt - This state applies only to neighbors on NBMA networks, where neighbors are manually configured.  A router sends packets to a neighbor in attempt state at the hello-interval instead of the poll-interval.

3.  Init - This state indicates that a hello packet has been seen from the neighbor in the last router dead-interval, but two-way communication has not yet been established.

4.  2Way - Indicates that the router has seen its own router ID in the neighbor field of the neighbor's hello packets, meaning bidirectional conversation has been established.  On multi-access networks, neighbors must be in this state or higher to be eligible to be elected as the DR or BDR.

5.  ExStart - The router and its neighbor will establish a master/slave relationship and determine the initial DD sequence number to exchange of Data Descriptor Packet's (DDP’s).  The neighbor with the highest router ID becomes the master.

6.  Exchange - The router sends DDP's describing in summary its entire link-state database to neighbors that are in the Exchange state.  The router may also send Link State Request packets, requesting more recent LSAs, to neighbors in this state.

7.  Loading - The router sends Link State Request packets to neighbors, requesting more recent LSAs that have been discovered in the exchange state but have not yet been received.

8.  Full - Neighbors in this state are fully adjacent, and the adjacencies appear in router LSAs and network LSAs.


- The adjacency building process uses four OSPF packet types

1.  DDP: Database Description packets (type 2) - Carry a summary description of each LSA in the originating router's link-state database.
       These descriptions are not the complete LSAs.   
         Three flags in the DD packet are used to manage the adjacency building   

         process:                                
        - I-bit, or Initial bit, when set indicates the first DD packetsent.                                                      
        - M-bit, or More bit, when set indicates that this is not the last DD packet to be sent.
        - MS-bit, or Master/Slave bit, is set in the DD packets originated by the master.


2.  LSR: Link State Request packets (type 3)
3.  LSU: Link State Update packets (type 4)
4.  LSAck: Link state Acknowledgement packets (type 5)

- All LSAs sent in update packets must be individually acknowledged, by one of two means:

1.  Explicit Acknowledgment - A Link State Acknowledgment packet containing the LSA header is received.

2.  Implicit Acknowledgment - A update packet that contains the same instance of the LSA is received.  Do not confuse LSA (Link State Advertisement) with LSAck (Link state Acknowledgement).