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Optimal path determination
The determination of the optimal path is based on different standards of measurement, for
example, path length, and metric. Routing algorithms calculate path indexes to determine the
optimal path to destination.
To facilitate the process of path determination, routing algorithms initialize and maintain routing
tables, which contain the routing information. This information changes depending on the routing
algorithm used.
Routing algorithms fill in routing tables with different information. “Destination/next hop”
combinations tell a router that a destination can be reached through the shortest path by sending
a packet to a particular router representing the “next hop” on the way to the final destination.
When the router receives an incoming packet, it checks the destination address and makes an
attempt to associate this address with a next hop. An example of a routing table is shown below.
Destination address Next hop
27 Router A
57 Router B
17 Router C
24 Router A
52 Router A
16 Router B
26 Router A
Routing table also contain other information. “Metrics” represent information about the desirability
of a path or a route. Routers compare metrics to determine the optimal routes. Metrics differ
depending on the routing algorithms being used. A variety of common metrics will be described
below in this chapter.
Routers communicate with each other (and maintain their routing tables) by transmitting various
messages. One of these messages is the “routing update”. The routing update usually includes
all or a part of a routing table. By analyzing routing update information from all routers, any router
can build a detailed picture of network topology. Another example of a message exchange
between routers is a “link-state advertisement”. Link state advertisements inform other routers
about sender’s link-states. Link information also can be used to build a full picture of network
topology. After the network topology is determined, routers can determine optimal paths to
destinations.
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