When a routing protocol algorithm updates the routing
table, the primary objective of the algorithm is to determine the best route to include in the table.
Each distance vector routing protocol uses a different routing metric to determine the best route.
The algorithm generates a number called the metric value for each path through the network. With
the exception of BGP, the smaller the metric, the better the path.
Metrics can be calculated based on a single characteristic of a path. More complex metrics can be
calculated by combining several path characteristics. The metrics that routing protocols most
commonly use are as follows:
?– Hop count: The number of times that a packet passes through the output port of one router
?– Bandwidth: The data capacity of a link; for instance, normally, a 10-Mbps Ethernet link is
preferable to a 64-kbps leased line
?– Delay: The length of time that is required to move a packet from source to destination
?– Load: The amount of activity on a network resource, such as a router or link
?– Reliability: Usually refers to the bit error rate of each network link
?– Cost: A con?¬?gurable value that on Cisco routers is based by default on the bandwidth of the
interface
Figure 3-3 shows an example of multiple routes between two hosts and the way different routing
protocols compute metrics.
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