Some disadvantages to static routes are that they require maintenance, and they require resources on routers. This can become significant with a large number of static routes. Routing protocols are dynamic. They learn about reachability within and between networks and update that information periodically; thus, they are needed when alternate paths exist in the network (i.e., when there is some degree of diversity). When there is only one path into or out of an area, a routing protocol cannot adapt to changes in topology. A stub network is a network with only one path into or out of it, as in Figure 6.23. With stub networks, a static route can be applied between the stub network and the network it is connected to. The tradeoff entailed in using static routes is not having to configure and maintain a routing protocol versus having to maintain the static routes. Since a stub network has only one path into or out of it, a default route is usually all that is needed. A static route may be used to provide this default. Static routes can also be used to force routing along a certain path. Since a routing protocol is not used, alternate paths are not learned or used, and thus traffic is forced to use the static route. There are times when this can be useful (e.g., it Network Stub Areas No Transient Traffic Network Network Network FIGURE 6.23 Stub Networks 284 CHAPTER 6 Addressing and Routing Architecture is sometimes used to enhance security). However, it should always be used with caution. Whenever there are multiple paths into and out of a network, then a routing protocol should be considered. Popular routing protocols, such as RIP, RIPv2, and OSPF, use either a distancevector or linkstate routing algorithm to determine reachability. In a distancevector routing algorithm each router maintains the “distance” (a metric to weight each hop, or connection between routers) between itself and possible destinations. A vector (or list) of these distances is computed from distance information received from other participating routers on that network. In a linkstate routing algorithm each router learns about itself, its links to nexthop routers (its neighbors), and the state of each link. This information is multicasted to other participating routers, and all routers build their routing information from the sum of these multicasts. For an excellent description of routing protocol algorithms, see Interconnections, second edition (the full reference is listed in Section 6.1.1). RIP and RIPv2 are IGPs that are based on a distancevector routing algorithm. This implies some characteristics of the dynamic behavior of RIPRIPv2routed networks. RIP and, to a lesser degree, RIPv2 are relatively straightforward to implement and maintain. RIP has been around for a long time (more than 30 years), having been part of the TCPIP protocol suite shipped with most UNIX systems, and there is a lot of experience with RIProuted networks.