3.6 Default Routing
In some situations, it is not desirable for all routers to have complete routing information. For example, in Figure 3-8, router R1 should know only about subnets 10.1.0.010.3.0.0; if it is sending a packet somewhere elsethe Internet or another company's networkit sends it via router Rc, which has a full routing table. The routing decision for router R1 would then be, "If I don't have information about a destination network in my routing table, I use the route to Rc." This type of routing information is called a default route. "Default" means that the route is used only if there is no more specific information about the destination network a packet is going to.
Figure 3-8. Principle of default routing
Use of default routes simplifies network management, as it saves a router from having to know all networks and makes it possible for routers to reference a "smarter guy" that can do the work. Default routes are widely used in real networks.
Like any other route, default routes can be either static or dynamic. The administrator can configure a static default route manually, saying, "For all other destinations, use router Rx." In case of a dynamically propagated default route, the administrator configures a router to originate it and say, "Everyone around, use me to reach any network you don't know about." After receiving such a routing update, other routers install the default route in their routing tables; the administrator doesn't need to configure it manually (Figure 3-9).
Figure 3-9. Dynamic propagation of a default route
A default route is usually represented as a pseudonetwork with all 0s in the address and mask parts, that is, 0.0.0.0 0.0.0.0 or 0.0.0.0/0. Most dynamic protocols use this convention to provide default routing information. IGRP, however, has its own method, considered in Chapter 8.
Default routes are similar to summary routes. A summary route to a major network hides all details of that network, saying, "Want to reach someone in this network? Go this way!" A default route hides all details about all networks, saying, "Want to reach someone somewhere? Come on over here!" The way routers treat summary and default routes, as well as the algorithm of router operations, depends on the kind of environmentclassful or classlessthe routers work in. The following sections cover the basic forwarding algorithm and the details of the routing table lookup performed in classful and classless modes.