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Embed code for: 23.6 Understanding Link-State Routing Protocols
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23.6 Understanding Link-State Routing Protocols
Understanding LinkState Routing Protocols Section 23: Implementing Dynamic Routing Linkstate routing protocols collect routing information from all other routers in the network or from routers within a defined area of the network. After all of the information is collected, each router, independent of the other routers, calculates the best paths to all destinations in the network. Because each router maintains its own view of the network, a router is less likely to propagate incorrect information that is provided by another router. A link is like an interface on a router. The state of the link is a description of that interface and of its relationship to its neighboring routers. An example of a description of the interface would include the IP address of the interface, the mask, the type of network to which it is connected, the routers that are connected to that network, and so on. The collection of link states forms a linkstate (or topological) database. The linkstate database is used to calculate the best paths through the network. Linkstate routers find the best paths to destinations by applying the Dijkstra algorithm against the linkstate database to build the SPF tree. The best paths are then selected from the SPF tree and placed in the routing table. Linkstate routing protocols are much more complex than their distance vector counterparts. However, the basic functionality and configuration of linkstate routing protocols are not complex at all. To maintain routing information, linkstate routing uses LSAs, a topological database, the SPF algorithm, the resulting SPF tree, and a routing table of paths and ports to each network. LSAs are used in OSPF in order for the routers to exchange knowledge about the network topology. The topological database holds all the information about the topology of the network. This database is used by the SPF algorithm, which builds the SPF tree. The best paths are then inserted into the routing table, and routing decisions are made based on the entries in the routing table. The ability of linkstate routing protocols, such as OSPF, to divide one large AS into smaller groupings of routers (called areas) is referred to as hierarchical routing. Link state routing protocols use the concept of areas for scalability. Topological databases contain information about every router and the associated interfaces, which, in large networks, can use resources intensively. Arranging routers into areas effectively partitions this potentially large database into smaller and more manageable databases. With hierarchical routing, routing still occurs between the areas (interarea routing). At the same time, many of the smaller internal routing operations, such as recalculating the database, are kept within an area. When a failure occurs in the network, such as a neighbor becoming unreachable, link state protocols flood LSAs by using a special multicast address throughout an area. Each linkstate router takes a copy of the LSA, updates its linkstate (topological) database, and forwards the LSA to all neighboring devices. LSAs cause every router within the area to recalculate routes. Because LSAs must be flooded throughout an area and all routers within that area must recalculate their routing tables, the number of link state routers that can be in an area should be limited. The figure shows three areas. If area 1 is having problems with an intermittent link, routers in the other areas do not need to continually run their SPF calculation, because they are isolated from the area 1 problem. Up Next: Introducing OSPF