A router operates at Layer 3 of the Open Systems Interconnection (OSI) reference model, the Network Layer. The device distinguishes among network layer protocols— such as IP, IPX, and AppleTalk—and makes intelligent packet delivery decisions using an appropriate routing protocol. Used on wired and wireless networks, routers can be used to segment a network with the goals of limiting broadcast traffic and providing security, control, and redundant paths.

A router also can provide multiple types of interfaces, including those for T1, Frame Relay, Integrated Services Digital Network (ISDN), Asynchronous Transfer Mode (ATM), cable networks, and Digital Subscriber Line (DSL) services, among others. Some routers can perform simple packet filtering to control the kind of traffic that is allowed to pass through them, providing a rudimentary firewall service. Larger routers can perform advanced firewall functions.

Arouter is similar to a bridge in that both provide filtering and bridging functions across the network. But while bridges operate at the Physical and Data Link Layers of the OSI reference model, routers join LANs at the Network Layer. Routers convert LAN protocols into WAN protocols and perform the process in reverse at the remote location. They may be deployed in mesh as well as point-to-point networks and, in certain situations, can be used in combination with bridges.

Although routers include the functionality of bridges, they differ from bridges in the following ways: They generally offer more embedded intelligence and, consequently, more sophisticated network management and traffic control capabilities than bridges. Another distinction—perhaps the most significant one—between a router and a bridge is that a bridge delivers packets of data on a “best effort” basis, specifically, by discarding packets it does not recognize onto an adjacent network.

Through a continual process of discarding unfamiliar packets, data get to theirs proper destination—on a network where the bridge recognizes the packets as belonging to a device attached to its network. By contrast, a router takes a more intelligent approach to getting packets to their destination— by selecting the most economical path (i.e., least number of hops) on the basis of its knowledge of the overall network topology, as defined by its internal routing table. Routers also have flow-control and error-protection capabilities.

Types of Routing

There are two types of routing: static and dynamic. In static routing, the network manager configures the routing table to set fixed paths between two routers. Unless reconfigured, the paths on the network never change. Although a static router will recognize that a link has gone down and issue an alarm, it will not automatically reroute traffic.

A dynamic router, on the other hand, reconfigures the routing table automatically and recalculates the most efficient path in terms of load, line delay, or bandwidth. In wired networks, some routers balance the traffic load across multiple access links, providing an N × T1 inverse multiplexer function. This allows multiple T1 access lines operating at 1.544 Mbps each to be used as a single higher-bandwidth facility.

If one of the links fails, the other links remain in place to handle the offered traffic. As soon as the failed link is restored to service, traffic is spread across the entire group of lines as in the original configuration.

Routing Protocols

Each router on the network keeps a routing table and moves data along the network from one router to the next using such protocols as the Open Shortest Path First (OSPF) protocol and the Routing Information Protocol (RIP). Although still supported by many vendors, RIP does not perform well in today’s increasingly complex networks. As the network expands, routing updates grow larger under RIP and consume more bandwidth to route the information.

When a link fails, the RIP update procedure slows route discovery, increases network traffic and bandwidth usage, and may cause temporary looping of data traffic. Also, RIP cannot base route selection on such factors as delay and bandwidth, and its line-selection facility is capable of choosing only one path to each destination. The newer routing standard, OSPF, overcomes the limitations of RIP and even provides capabilities not found in RIP.

The update procedure of OSPF requires that each router on the network transmit a packet with a description of its local links to all other routers. On receiving each packet, the other routers acknowledge it, and in the process, distributed routing tables are built from the collected descriptions. Since these description packets are relatively small, they produce a minimum of overhead. When a link fails, updated information floods the network, allowing all the routers to simultaneously calculate new tables.

Types of Routers

Multiprotocol nodal, or hub, routers are used for building highly meshed internetworks. In addition to allowing several protocols to share the same logical network, these devices pick the shortest path to the end node, balance the load across multiple physical links, reroute traffic around points of failure or congestion, and implement flow control in conjunction with the end nodes.

They also provide the means to tie remote branch offices into the corporate backbone, which might use such WAN services as Transmission Control Protocol/Internet Protocol (TCP/IP), T1, ISDN, and ATM. Access routers are typically used at branch offices. These are usually fixed-configuration devices available in Ethernet and Token Ring versions that support a limited number of protocols and physical interfaces.

They provide connectivity to high-end multiprotocol routers, allowing large and small nodes to be managed as a single logical enterprise network. Although low-cost, plug-and-play bridges can meet the need for branch office connectivity, low-end routers can offer more intelligence and configuration flexibility at comparable cost. The newest access routers are multiservice devices that are designed to handle a mix of data, voice, and video traffic.

They support a variety of WAN connections through built-in interfaces that include dual ISDN Basic Rate Interface (BRI) interfaces, dual analog ports, T1/Frame Relay ports, and an ISDN interface for videoconferencing. Such routers can run software that provides Internet Protocol Secure (IPSec) virtual private network (VPN), firewall, and encryption services.

Midrange routers provide network connectivity between corporate locations in support of workgroups or the corporate intranet, for example. These routers can be stand-alone devices or packaged as modules that occupy slots in an intelligent wiring hub or LAN switch. In fact, this type of router is often used to provide connectivity between multiple wiring hubs or LAN switches over high-speed LAN backbones such as ATM, Fiber Distributed Data Interface (FDDI), and Fast Ethernet.

There is a consumer class of routers for 2.4-GHz Wireless Fidelity (Wi-Fi) networks that are capable of providing shared access to the Internet over such broadband technologies as cable and DSL. The EtherFast Wireless AP + Cable/DSL Router from Linksys, for example, connects a wireless network to a high-speed broadband Internet connection and a 10/100 Fast Ethernet backbone.

Configurable through any networked PC’s Web browser, the router can be set up for Network Address Translation (NAT), allowing it to act as an externally recognized Internet device with its own IP address for the home LAN. The Linksys device is also equipped with a four-port Ethernet switch. The combination of wireless router and switch technology eliminates the need to buy an additional hub or switch and extends the range of the wireless network.

Whether used on a wired or wireless network or a hybrid network, routers fulfill a vital role in implementing complex mesh networks such as the Internet and private intranets using Layer 3 protocols, usually IP. They also have become an economical means of tying branch offices into the enterprise network and providing PCs tied together on a home network with shared access to broadband Internet services such as cable and DSL.

Like other interconnection devices, enterprise-class routers are manageable via SNMP, as well as the proprietary management systems of vendors. Just as bridging and routing functions made their way into a single device, routing and switching functions are being combined in the same way.