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How routing information is maintained

How routing information is maintained

2.1.3 This page will explain how link-state protocols use the following features:
  • The LSAs
  • A topological database
  • The SPF algorithm
  • The SPF tree
  • A routing table of paths and ports to determine the best path for packets 
Link-state routing protocols were designed to overcome the limitations of distance vector routing protocols. For example, distance vector protocols only exchange routing updates with immediate neighbors while link-state routing protocols exchange routing information across a much larger area.
When a failure occurs in the network, such as a neighbor becomes unreachable, link-state protocols flood LSAs with a special multicast address throughout an area. This process sends information out all ports, except the port on which the information was received. Each link-state router takes a copy of the LSA and updates its link-state, or topological database. The link-state router then forwards the LSA to all neighbor devices. LSAs cause every router within the area to recalculate routes. For this reason, the number of link-state routers within an area should be limited.
A link is the same as an interface on a router. The state of the link is a description of an interface and the relationship to the neighbor routers. For example, a description of the interface would include the IP address of the interface, the subnet mask, the type of network that it is connected to, the routers connected to that network, and so on. The collection of link-states form a link-state database which is sometimes called a topological database. The link-state database is used to calculate the best paths through the network. Link-state routers apply the Dijkstra shortest path first algorithm against the link-state database. This builds the SPF tree with the local router as the root. The best paths are then selected from the SPF tree and placed in the routing table.
The next page will discuss the link-state routing algorithm.

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