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Cisco Network Topology and Design

Chapter Description

Explore design issues related to overall network topology with this sample chapter from CCIE Professional Development: Large-Scale IP Network Solutions by Cisco Press.

Hierarchy Issues

Just as the Internet hierarchy had to adapt to accommodate an increase in players, so has the enterprise network. Early data networks facilitated the basic requirement of communication between terminals and a central mainframe. The need for a simple hierarchy, from terminal to IBM cluster controllers to front-end processors, was readily apparent from this basic requirement. In today's world of peer-to-peer networking, however, the reasons for network hierarchy and its inner workings are subtler, yet just as important for successful network design.

Figure 4-2 presents the high-level network architecture developed in Chapter 1. Notice that the network backbone (also called the network core—the terms core and backbone are equivalent) consists of mesh-connected backbone routers that reside at distribution centers (DCs) within each service region.

Each DC, which may house a LAN topology that is resilient to single node failure, forms the hub of a star distribution network for that region. Finally, the access network consists of both provider and customer premise equipment, which is typically homed to one or more access POPs.

Figure 4-2 Modularization of a Large Network

In the example in Figure 4-2, only three regions/DCs and, at most, three POPs in a region are shown. However, the hierarchical model will scale much more than this using current commercial routers.

A typical large IP network, whether an ISP or a large corporate intranet, will consist of routers performing a number of different roles. It is convenient to define three major roles corresponding to each layer in the hierarchy: backbone, distribution, and access.

As shown in Figure 4-3, which illustrates the arrangement in a typical high-resilience DC, these roles possess a specific hierarchical relationship. Backbone routers are at the top, distribution routers are in the middle, and access routers are at the bottom.

Figure 4-3 Distribution Center Architecture

Backbone Routers

The backbone routers core1.sfo and core2.sfo reside in the San Francisco DC and are responsible for connecting the regional network to the backbone. These routers forward packets to and from the region. They also advertise reachability for that region, either to the core routers of other regions (in other major cities), or to external peer networks (other ISPs).

Backbone routers are also peers in terms of the useful reachability information they possess. This does not imply that router core1.sfo has the same detailed topological information about Los Angeles as, say, router core1.lax, but it does indicate that core1.sfo understands that core1.lax and core2.lax, rather than core1.stl, are the gateways to all destinations in the Los Angeles region.

Backbone routers contain reachability intelligence for all destinations within the network. They possess the capability to distinguish between the gateway information and the information that explains how to reach the outside world, which is through other peer networks or the Internet.

Distribution Routers

Distribution routers consolidate connections from access routers. They are often arranged in a configuration that is resilient to failure of a single core router. Distribution routers usually contain topological information about their own region, but they forward packets to a backbone router for inter-region routing.


In smaller regions, distribution and backbone routers may be one and the same. In larger regions, distribution routers themselves may form a hierarchy.

High-performance customers on permanent WAN links often may connect directly to distribution routers, whereas dial-on-demand customers typically do not because this would impose the need to run dial-authentication software images of distribution routers.

Access Routers

Access routers connect the customer or enterprise site to the distribution network. In the ISP case, the router at the remote end of an access link is typically the customer premises equipment, and may be owned and operated by the customer.

For large enterprise networks, in which the LANs and WANs are managed by different divisions or contractors, the access router typically is managed by either the WAN or the LAN operator—usually this is the latter if the LAN is very large.

You now may wonder: Why is it important to distinguish between the backbone, access, and distribution routers? The reason is that they are increasingly becoming very distinct hardware/software combinations. In access routers, for example, you already have seen the need for support of dial-on-demand and authentication, as well as route filtering and packet filtering and classification.

In distribution routers, the emphasis is on economical aggregation of traffic and the support of varied media WAN types and protocols. In backbone routers, the emphasis is on supporting extremely high speeds, and aggregation of a very limited set of media types and routing protocols. These differences are summarized in Table 4-1.

Table 4-1 Characteristics of Backbone, Distribution, and Access Routers

Router Type


Backbone router

Scalable: packet forwarding, WAN links, QoS, routing Expensive Redundant WAN links National infrastructure

Distribution router

Scalable: WAN aggregation, LAN speeds Redundant LAN links Less expensive

Access router

Scalable: WAN aggregation Cheap Complex routing/QoS policy setting, access security, and monitoring capabilities

This discussion focused attention on the WAN environment and has avoided any issues of LAN design, other than the use of specific LAN technology within the distribution or access networks. In particular, at the individual user or network host level, access technologies include ATM, FDDI, Token Ring, or the ubiquitous Ethernet; rather than such technologies as Frame Relay, T1, SMDS, and SONET.

Scaling LANs through the use of hierarchy is itself the subject of much literature. To study this area further, interested readers should refer to the references listed at the end of this chapter.

The origins of the three-tiered, backbone-distribution-access hierarchy can be traced to the evolution of the Internet (refer to Chapter 1). However, hierarchical design is certainly nothing new and has been used in telephone networks and other systems for many years. In the case of IP data networking, there are several reasons for adding hierarchy.

Not only does hierarchy allow the various elements of routing, QoS, accounting, and packet switching to scale; but it also presents the opportunity for operational segmentation of the network, simpler troubleshooting, less complicated individual router configurations, and a logical basis for distance-based packet accounting.

These issues are examined in great depth in Part II of this book, "Core and Distributing Networks." For the moment, we will examine the topologies used within the backbone, distribution, and access layers of the network architecture.

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