If the routing table is a list of all the possible paths a packet can take, the forwarding table is a list of only the best routes to a particular destination. The best path is determined according to the particular routing protocol being used, but generally the number of hops between the source and destination determines the best possible route.
In the network shown in Figure 45, because the path with the fewest number of hops from San Francisco to Miami is through Phoenix, the forwarding table distills all the possible San Francisco-Miami routes into the single route through Phoenix. All traffic with a destination address of Miami is sent directly to the next hop, Phoenix.
After it receives a packet, the Phoenix router performs another route lookup, using the same destination address. The Phoenix router then routes the packet appropriately. Although it considers the entire path, the router at any individual hop along the way is responsible only for transmitting the packet to the next hop in the path. If the Phoenix router is managing its traffic in a particular way, it might send the packet through Houston on its route to Miami. This scenario is likely if specific customer traffic is treated as priority traffic and routed through a faster or more direct route, while all other traffic is treated as nonpriority traffic.