Issues and trends in router design

outers knit together the constituent networks of the global Internet, creating the illusion of a unified whole. While their primary role is to transfer packets from a set of input links to a set of output links, they must also deal with heterogeneous link technologies, provide scheduling support for differential service, and participate in complex distributed algorithms to generate globally coherent routing tables. These demands, along with an insatiable need for bandwidth in the Internet, complicate their design. Routers are found at every level in the Internet. Routers in access networks allow homes and small businesses to connect to an Internet service provider (ISP). Routers in enterprise networks link tens of thousands of computers within a campus or an enterprise. Routers in the backbone are not usually directly accessible to end systems. Instead, they link together ISPs and enterprise networks with long distance trunks. The rapid growth of the Internet has created different challenges for routers in backbone, enterprise, and access networks. The backbone needs routers capable of routing at high speeds on a few links. Enterprise routers should have low cost per port and a large number of ports, be easy to configure, and support quality of service (QoS). Finally, access routers should support many heterogeneous high-speed ports and a variety of protocols at each port, and try to bypass the central office voice switch. This article presents the design issues and trends that arise in these three classes of routers. The following section describes the structure of a generic router. The section after that discusses design issues in backbone, enterprise, and access routers. We then present some recent advances and trends in router design. Finally, we conclude with a description of some open problems. We note that our main topic of discussion is packet forwarding; routing protocols, which create the forwarding tables, are dealt with only in passing. Figure 1 abstracts the architecture of a generic router. A generic router has four components: input ports, output ports, a switching fabric, and a routing processor. An input port is the point of attachment for a physical link and is the point of entry for incoming packets. Ports are instantiated on line cards, which typically support 4, 8, or 16 ports. The switching fabric interconnects input ports with output ports. We classify a router as input-queued or output-queued depending on the relative speed of the input ports and the switching …