Distribution and hierarchically superior routers have a more general view of alternate paths, although even these routers can default to a more knowledgeable router. Default route assumptions can even say "if the destination isn't within my network, go to the Internet gateway and let the ISP know how to get there." Losing the default can cause a network to be isolated.

When you do need dynamic routing with a generic IGP, avoid the Routing Information Protocol because failover can take several minutes in a network of appreciable size. Generally, use Open Shortest Path First (OSPF).

As networks grow, the workload on such protocols as OSPF grow exponentially with the number of subnets and linearily with the number of routers. Eventually, scalable router networks need to include hierarchy even in their IGPs, much less their Internet connectivity.

Route discovery with BGP

Very large enterprise networks may need to interconnect several internal hierarchies, such as continental domains for a worldwide company. To interconnect internal networks, the Border Gateway Protocol (BGP) is often needed. BGP is also the basic protocol for any nontrivial connection to the Internet.