Default routes can be configured either on each router that needs a default route or on one router that in turn advertises the routes to its peers. The case studies of this section examine both methods.
Recall from the discussion in Chapter 5, “Routing Information Protocol (RIP),” of classful route lookups that a router will first match a major network number and then match the subnet. If a subnet cannot be matched, the packet will be dropped. Classful route lookup is the default behavior on Cisco routers; lookups can be changed to classless (even for classful routing protocols) with the global command ip classless.
Routers using a default route should perform classless lookups.
Any router using a default ...