The metric assigned to each network depends on the type of protocol. Some simple protocol, like RIP, treats each network as equals. The ______ of passing through each network is the same; it is one ______ count. So if a packet passes through 10 network to reach the destination, the total cost is 10 hop counts. Other protocols, such as OSPF, allow the administrator to assign a cost for passing through a network based on the type of service required.A ______ through a network can have different costs (metrics). For example, if maximum ______ is the desired type of service, a satellite link has a lower metric than a fiber-optic line. On the other hand, if minimum ______ is the desired type of service, a fiber-optic line has a lower metric than a satellite line. OSPF allow each router to have several routing table based on the required type of service.
A．bandwidth
B．efficiency
C．availability
D．delay
参考答案：D
解析： 赋予每一个网络的路由度量依赖于协议的类型。诸如RIP之类的简单协议，可以认为每个网络都是相同的，因而通过每一个网络的费用(cost)也都是相同的，即1跳(hop)。因此，如果一个分组到达目的地需要经过10个网络，则其总的费用就是10跳。其他的协议，例如OSPF，允许网络管理员根据要求的服务类型赋予所通过的网络一个度量值。通过一个网络的路由(route)可以具有不同的费用(或度量值)。例如，若期望的服务类型为最大吞吐率(throughput)，则卫星链路比光纤线路的费用(cost)低。而如果期望的服务类型为最小延迟(delay)，则光纤线路比卫星线路的费用(cost)低。OSPF协议允许每一个路由器根据需要的服务类型设置几个不同的路由表。