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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