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.
|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.
|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|