Q & A on Routing updates Q: You had mentioned that one of the drawbacks of var. length subnetting is that it would make the routing tables large and there is more computational effort going on at the time of routing updates. What is a routing update ? Why would one router want to exchange routing table information with another ? A: When a router in the Internet receives a datagram, it must forward the datagram on toward its destination. Thus, routers need to know the shortest path to all destinations, and must have the information stored in a routing table (which the forwarding software uses). The shortest paths are found "automatically" -- routers run routing update protocols that pass the information around. One technique involves pairs of routers exchanging their routes periodically (known by the name "Distance Vector").