Single Area OSPF (page 1)
26 Oct 2003 @ 11:18AM
by Satis

Updated: 20 Jan 2010 @ 09:08AM
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.
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A router running OSPF maintains several databases as listed below:
Link-State Database(topological database)State of all router links across an area. Should be identical across an area.
Forwarding DatabaseSPF algorithm run on links in the link-state database. This is different for every router.
Adjacencies Database(routing table)Keeps track of neighbor routers.
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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.
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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 TypeNameDescription
1HelloEstablishes and maintains adjacency information with neighbor routers.
2Database Description Packet(DBD)Describes the contents of an OSPF router's link-state database.
3Link-State Request(LSR)Requests a specific piece of a router's link-state database.
4Link-State Update(LSU)Transports link state advertisements (LSAs) to neighbor routers.
5Link-State Acknowledgement(LSAck)Acknowledges receipt of a neighbor's Link-State Advertisement.
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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 StateDescriptionPacket 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
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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 TypeExampleDR/BDR?
Broadcast Multiaccessethernet, token ring, FDDIyes
Nonbroadcast Multiaccessframe relay, x.25, SMDSyes
Point to PointPPP, HDLCno
Point to Multipoint Administrator-definedno
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