10.2 | IP Routing Protocols | |||
10.2.4 | Path determination |
Path determination occurs at the network
layer.
Path determination enables a router to compare the destination address
to the available routes in its routing table, and to select the best
path. The routers learn of these available routes through static
routing or dynamic routing. Routes configured manually by the network
administrator are static routes. Routes learned by others routers
using a routing protocol are dynamic routes.
The router uses path determination to decide which port an incoming packet should be sent out of to travel on to its destination. This process is also referred to as routing the packet. Each router that the packet encounters along the way is called a hop. The hop count is the distanced traveled. Path determination can be compared to a person driving a car from one location in a city to another. The driver has a map that shows the streets that can be taken to get to the destination, just as a router has a routing table. The driver travels from one intersection to another just as a packet travels from one router to another in each hop. At any intersection, the driver can route himself by choosing to turn left, turn right, or go straight ahead. In the same manner, a router decides which outbound port the packet should be sent. The decisions of a driver are influenced by factors such as traffic on a road, the speed limit of the road, the number of lanes on the road, whether or not there is a toll on the road, and whether or not the road is frequently closed. Sometimes it is faster to take a longer route on a smaller, less crowded back street instead of a highway with a lot of traffic. Similarly, routers can make decisions based on the load, bandwidth, delay, cost, and reliability of a network link. The following process is used during path determination for every packet that is routed:
|