|  | 11.2 | Why Routing Protocols are Necessary
| 11.2.6 |
How distances on network paths are determined by various metrics
When a routing algorithm updates a routing table, its primary objective is to determine the best information to include in the table. Each routing algorithm interprets what is best in its own way. The algorithm generates a number, called the metric value, for each path through the network. Typically, the smaller the metric number, the better the path. You can calculate metrics based on a single characteristic of a path; you can calculate more complex metrics by combining several characteristics. The metrics most commonly used by routers are as follows: bandwidth -- the data capacity of a link; (normally, a 10 Mbps Ethernet link is preferable to a 64 kbps leased line) delay -- the length of time required to move a packet along each link from source to destination load -- the amount of activity on a network resource such as a router or link reliability -- usually refers to the error rate of each network link hop count -- the number of routers a packet must travel through before reaching its destination ticks -- the delay on a data link using IBM PC clock ticks (approximately 55 milliseconds). cost -- an arbitrary value, usually based on bandwidth, monetary expense, or other measurement, that is assigned by a network administrator