Single Area OSPF is a routing protocol that uses the Shortest Path First (SPF) algorithm (also know as the Dijkstra algorithm) to making routing decisions. It is a link-state routing protocol with a hierarchical design, enabling it to scale effectively. Because it is link-state, every router has a complete picture of the network topology from which to base routing decisions. Additionally, routing updates are only sent if there is a topology change, thereby reducing packet overhead.

A router running OSPF maintains several databases as listed below:

Database	Description
Link-State Database(topological database)	State of all router links across an area. Should be identical across an area.
Forwarding Database	SPF algorithm run on links in the link-state database. This is different for every router.
Adjacencies Database(routing table)	Keeps track of neighbor routers.

A Designated Router (DR) becomes adjacent to all routers in a multiaccess network and becomes a spokesperson for that multiaccess network. A Backup Designated Router(BDR) is elected at the same time in case the DR goes down. DRs and BDRs only exist on multiaccess networks.

There are 5 different packet types used by OSPF. These discover neighbor routers, form adjacencies, transfer routing data, and are used as keepalives to discover link failures. The five types are listed in the table below.

Packet Type	Name	Description
1	Hello	Establishes and maintains adjacency information with neighbor routers.
2	Database Description Packet(DBD)	Describes the contents of an OSPF router's link-state database.
3	Link-State Request(LSR)	Requests a specific piece of a router's link-state database.
4	Link-State Update(LSU)	Transports link state advertisements (LSAs) to neighbor routers.
5	Link-State Acknowledgement(LSAck)	Acknowledges receipt of a neighbor's Link-State Advertisement.

When OSPF is first initialized on a router, it goes through 7 states, from no connection to full adjacency. These steps are listed below.

OSPF State	Description	Packet Used
1. Down State	No information has yet been exchanged.	none
2. Init State	Routers send hello packets at regular intervals (default 10 seconds) to establish relationships. Any hello packets received from other routers are noted and the ip address included in future hello packets.	Hello
3. Two-way State	A router sees itself in a hello packet.	Hello
4. ExStart	Routers negotiate master/slave relationship by comparing their router id using hello packets. The highest interface ip address is used to determine the master (including loopbacks).	Hello
5. Exchange State	Neighbors use type 2 (DBD) packets to send each other link-state information. If a router receives information about a new link, it requests a complete update.	DBD
6. Loading State	After link-state database descriptions are sent, a router may request more complete information using type 3 packets (LSRs). The other router responds with a type 4 (LSU) packet containing the link-state information. Finally, the LSUs are acknowledged with a type 5 (LSAck) packet.	LSR, LSU, LSAck
7. Full adjacency	Routers are fully adjacent.	all

There are four different network types as far as OSPF is concerned. Depending on which network it believes it resides on will make a difference as to how it operates.

Network Type	Example	DR/BDR?
Broadcast Multiaccess	ethernet, token ring, FDDI	yes
Nonbroadcast Multiaccess	frame relay, x.25, SMDS	yes
Point to Point	PPP, HDLC	no
Point to Multipoint	Administrator-defined	no