确定路由的度量METRIC-CCNA中文版PPT下载

确定路由的度量（METRIC） * Emphasize: How the routing algorithm defines “best” determines the most important characteristics of each routing algorithm. Hop count—Some routing protocols use hop count as their metric. Hop count refers to the number of routers a packet must go through to reach a destination. The lower the hop count, the better the path. Path length is used to indicate the sum of the hops to a destination. As indicated in the figure, RIP uses hop count for its metric. Ticks—Metric used with Novell IPX to reflect delay. Each tick is 1/18th of a second. Cost—Factor used by some routing protocols to determine the best path to a destination; the lower the cost, the better the path. Path cost is the sum of the costs associated with each link to a destination. Bandwidth—Although bandwidth is the rating of a link’s maximum throughput, routing through links with greater bandwidth does not always provide the best routes. For example, if a high-speed link is busy, sending a packet through a slower link might be faster. As indicated in the figure with highlighting, delay and bandwidth comprise the default metric for IGRP. Delay—Depends on many factors, including the bandwidth of network links, the length of queues at each router in the path, network congestion on links, and the physical distance to be traveled. A conglomeration of variables that change with internetwork conditions, delay is a common and useful metric. As indicated in the figure with highlighting, delay and bandwidth comprise the default metric for IGRP. Load—Dynamic factor can be based on a variety of measures, including CPU use and packets processed per second. Monitoring these parameters on a continual basis can itself be resource intensive.