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Link State

Routers using link state–based routing protocols exchange link state advertisements throughout the internetwork to update routing tables. Link state router advertisements consist of a router's attached network IDs and are advertised upon startup and when changes in the internetwork topology are sensed. Link state updates are sent using directed or multicast traffic rather than broadcasting. Link state routers build a database of link state advertisements and use the database to calculate the routing table. Routing information exchanged between link state–based routers is synchronized and acknowledged. Table 1.2 lists some link state routing protocols.

Table   1.2 Link StateBased Routing Protocols

Routable Protocol

Link State–Based Routing Protocol

IP

Open Shortest Path First (OSPF)

IPX

NetWare Link Services Protocol (NLSP)

Advantages of Link State–Based Routing Protocols

  • Smaller routing tables.
    Only a single optimal route for each network ID is stored in the routing table.

  • Low network overhead.
    Link state–based routers do not exchange any routing information when the internetwork has converged.

  • Ability to scale.
    Between the smaller routing tables and low overhead, link state–based routing protocols scale well to large and very large internetworks.

  • Lower convergence time.
    Link state–based routing protocols have a much lower convergence time and the internetwork is converged without routing loops.

Disadvantages of Link State–Based Routing Protocols

  • Complex.
    Link state–based routing protocols are much more complex and difficult to understand than distance vector–based routing protocols.

  • More difficult to configure.
    A link state–based routing protocol implementation requires additional planning and configuration.

  • Resource intensive.
    For very large internetworks, the database of link state advertisements and the calculation of routing table entries can be memory and processor intensive.

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