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OSPF terminology


OSPF terminology
2.2.2 There are many words and concepts for students in this TI and the figures should help to explain them. Use the interactive media activity to reinforce the terms and their abbreviations. Instructors might like to hold an acronym competition to see who can explain the words and concepts in the following table:
Link
A link is a physical and electrical connection between two network devices.
Link-state (LS)
Link-state is the status of a link between two routers. This status includes information about a router interface and its relationship to neighboring routers.
Cost
Cost is the value assigned to a link. Link-state protocols assign a cost to a link, which is based on the speed of the network connection.
Area
An area is a collection of networks and routers that has the same area identification. Each router within an area has the same link-state information. A router within an area is called an internal router.
Designated Router (DR)
A DR is one router on an OSPF multi-access network that represents all the routers in that network. Each OSPF network has a DR and BDR. These routers have special responsibilities that are discussed later in this module.
Backup Designated Router (BDR)
A BDR is a standby router that becomes the DR, if the original DR fails.
Adjacencies database (AD)
An AD is a listing of all the neighbors to which a router has established bi-directional communication.
Link-state database (LSD) or topological database
An LSD is a list of information about all other routers in the network. It shows the network topology. All routers within an area have identical link-state databases.
Routing table
The routing table, also known as the forwarding database, is generated when an algorithm is run on the link-state database. Each routing table is unique and contains information of how and where to send packets to other routers.
SPF algorithm
An SPF algorithm is a routing algorithm that iterates on length of path to determine a shortest-path spanning tree.
Link-state advertisement (LSA)
An LSA is a broadcast packet used by link-state protocols that contain information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables.


Link-state routers identify neighboring routers and then communicate with the identified neighbors. OSPF has its own terminology. The new terms are shown in Figure .
OSPF gathers information from neighbor routers about the link status of each OSPF router. This information is flooded to all its neighbors. An OSPF router advertises its own link-states and passes on received link-states.
The routers process the information about link-states and build a link-state database. Every router in the OSPF area will have the same link-state database. Therefore, every router has the same information about the state of the links and the neighbors of every other router.
Each router then applies the SPF algorithm on its own copy of the database. This calculation determines the best route to a destination. The SPF algorithm adds up the cost, which is a value that is usually based on bandwidth. The lowest cost path is added to the routing table, which is also known as the forwarding database.
Each router keeps a list of adjacent neighbors, called the adjacency database. The adjacency database is a list of all the neighbor routers to which a router has established bidirectional communication. This is unique to each router.
To reduce the number of exchanges of routing information among several neighbors on the same network, OSPF routers elect a designated router (DR) and a backup designated router (BDR) that serve as focal points for routing information exchange.
The Interactive Media Activity will teach students about OSPF terminology.
The next page will compare OSPF to distance vector protocols.


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