Border Gateway Protocol(BGP) is inter-autonomous system ______ protocol. BGP is based ona routing method called path vector routing. Distance vector routing is not a good candidate for inter-autonomoussystem routing because there are occasions on which the route with the smallest ______ count is not thepreferred route. For example, we may not want a packet through an autonomous system that is not secure eventhough it is shortest route. Also, distance vector routing is unstable due to the fact that the routers announce onlythe number of hop counts to the destination without defining the path that leads to that ______. A router thatreceives a distance vector advertisement packet may be fooled if the shortest path is actually calculated throughthe receiving router itself. Link ______ routing is also not a good candidate for inner-autonomous system routingbecause an internet is usually too big for this routing method. To use link state routing for the whole internetwould require each router to have a huge link state database. It would also take a long time for each router tocalculate its routing ______ using the Dijkstra algorism.
A．table
B．state
C．metric
D．cost
参考答案：A
解析：
[分析]： 边界网关协议(BGP)是自治系统间的路由协议。BGP是基于路由的方法，称为路径矢量路由。距离矢量路由不是自治系统间路由很好的候选者，因为其最小跳数的路由不一定是最合适的路由。例如，我们可能不希望数据包通过一个不安全的自治系统，即使它是最短的路线。此外，距离矢量路由是不稳定的，因为路由器只宣布到目的地的跳数，不指出到达目的地的路径。实际上，如果是通过接收路由器本身计算的最短路径，接收距离矢量通告报文的路由器可能被愚弄。链路状态路由在跨自治系统中也不是一个好的候选者，因为采用这种路由方法的互联网通常过大。要对整个互联刚使用链路状态路由，就需要每个路由器有一个巨大的链路状态数据库。它也需要每个路由器用很长的时间来使用Dijkstra算法计算自己的路由表。