The Link State Database

In addition to flooding LSAs and discovering neighbors, a third major task of the link state routing protocol is establishing the link state database. The link state or topological database stores the LSAs as a series of records. Although a sequence number and age and possibly other information are included in the LSA, these variables exist mainly to manage the flooding process. The important information for the shortest path determination process is the advertising router's ID, its attached networks and neighboring routers, and the cost associated with those networks or neighbors. As the previous sentence implies, LSAs may include two types of generic information:»21

[12] Actually, there can be more than two types of information and multiple types of link state packets. They are covered in the chapters on specific protocols.

• Router link information advertises a router's adjacent neighbors with a triple of ( Router ID, Neighbor ID, Cost), where cost is the cost of the link to the neighbor.

• Stub network information advertises a router's directly connected stub networks (networks with no neighbors) with a triple of (Router ID, Network ID, Cost).

The shortest path first (SPF) algorithm is run once for the router link information to establish shortest paths to each router, and then stub network information is used to add these networks to the routers. Figure 4.11 shows an internetwork of routers and the links between them; stub networks are not shown for the sake of simplicity. Notice that several links have different costs associated with them at each end. A cost is associated with the outgoing direction of an interface. For instance, the link from RB to RC has a cost of 1, but the same link has a cost of 5 in the RC to RB direction.

Figure 4.11. Link costs are calculated for the outgoing direction from an interface and do not necessarily have to be the same at all interfaces on a network.

Figure 4.11. Link costs are calculated for the outgoing direction from an interface and do not necessarily have to be the same at all interfaces on a network.

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