Border Gateway Protocol (BGP) is inter-autonomous system (71) protocol. BGP is based on a routing method called path vector routing. Distance vector routing is not a good candidate for inter-autonomous system routing because there are occasions on which the route with the smallest (72) count is not the preferred route. For example, we may not want a packet through an autonomous system that is not secure even though it is the shortest route. Also, distance vector routing is unstable due to the fact that the routers announce only the number of hop counts to the destination without actually defining the path that leads to that (73) . A router that receives a distance vector advertisement packet may be fooled if the shortest path is actually calculated through the receiving router itself. Link (74) routing is also not a good candidate for inter-autonomous system routing because an internet is usually too big for this routing methoD. To use link state routing for the whole internet would require each router to have a huge link state database. It would also take a long time for each router to calculate its routing (75) using the Dijkstra algorism.
（71）是（）
A.routing
B.switching
C.transmitting
D.receiving
展开为x的幂级数，其收敛域为（）。
