The Importance of Routing
Although Chapter 14, “Campus Design Models,” discusses the advantages of routing from a design perspective, it is also useful to consider the importance of routing when discussing Layer 3 switching—after all, Layer 3 switching is routing. This section is designed to serve only as a brief reminder of routing’s benefits in a large network (campus or WAN). For a more thorough discussion of this subject, please see the section “Advantages of Routing” in Chapter 14.
Probably the most important benefit of routing is its proven history of facilitating large networks. Although the Internet serves as the obvious example here, this point is true of any type of network, such as a large campus backbone. Because routers prevent broadcast propagation and use more intelligent forwarding algorithms than bridges, routers provide much more efficient use of bandwidth. This simultaneously results in flexible and optimal path selection.
For example, it is very easy to implement load balancing across multiple paths in most networks when using routing. On the other hand, as Chapter 7, “Advanced Spanning Tree,” discussed, Layer 2 load balancing can be very difficult to design, implement, and maintain. The data forwarding benefits of routers are especially important when multicast traffic is in use. As multicast traffic becomes increasingly common in campus networks, routers play an increasingly important role.
Routers provide additional benefits that reach beyond the area of data forwarding. Because Layer 3 addresses are hierarchical, routers can be used to implement summarized designs. By reducing routing protocol overhead, increasing table lookup performance, and improving network stability, this can further facilitate networks of almost unlimited size. Most routers provide extensive access list capabilities that can be used to provide important policy controls. Finally, routers can also provide important features such as DHCP relay, proxy Address Resolution Protocol (ARP), and Get Nearest Server (GNS) functions in IPX networks.
You should build routing (Layer 3 switching) into all but the smallest campus networks. See Chapters 14, “Campus Design Models,” and 15,”Campus Design Implementation,” for more information.