Forwarding involves looking up addresses up on routing tables and making forwarding decisions. Discovery is what builds those routing tables, and network managers can make their lives easier by streamlining this process. Large router networks like the Internet employ topology discovery as a matter of course. These networks involve millions of host systems and subnets, and tens of thousands of different network paths. Replacing discovery with manual control would generate so many errors that traffic would never get from one place to another.
Discovery is necessary, but it can be costly.
Routers exchange periodic updates to refresh their view of the network's structure and the location and status of subnets representing groups of users. These updates consume bandwidth and router processing time. Though topology discovery is critical to big IP networks, it is not necessarily critical or even useful to all networks. But discovery is only valuable in networks where routes and subnets vary. With a bit of work up front, managers may be able to control both these factors and do away with the need for topology discovery.