Bay Technical Associates RM356 manual Basic Router Concepts, What is a Router?

Reference Guide for the Model RM356 Modem Router

Basic Router Concepts

In general, the cost of providing network bandwidth is proportional to the data speed and the distance over which the network extends. Large amounts of bandwidth are provided easily and relatively inexpensively in a local area network (office, department, and similar situations). However, providing the same high data speeds between two local networks that are physically distant may be prohibitively expensive. Because of this expense, high-speed local area networks (LANs) are usually interconnected by slower-speed links to form a wide area network (WAN).

In order to make the best use of the slower WAN links, a mechanism must be in place at each location for selecting data meant only for another location and sending it by the best available link. The function of selecting and forwarding this data is performed by a router.

What is a Router?

A router is a device that forwards traffic between networks based on network layer information in the data and on routing tables maintained by the router. In these routing tables, a router builds up a logical picture of the overall network by gathering and exchanging information with other routers in the network. Using this information, it chooses the best path for forwarding network traffic.

Routers vary in performance and scale, number of routing protocols supported, and types of physical WAN connections supported. The Model RM356 Modem Router is a small office router that routes the IP protocol over a dial-up telephone connection.

Routing Information Protocol

One of the protocols used by a router to build and maintain a picture of the network is the Routing Information Protocol (RIP). RIP is a distance vector protocol, meaning that all the decisions about which path to use are based upon a logical distance between source and destination networks. This distance is measured in “hops,” meaning the number of relaying routers in the path between the source LAN router and the router of the destination LAN. For example, the LAN of router A is considered to be 1 hop away. If router A can reach the network of router B by a direct WAN link to the network of router B, the network of router B is two hops away. If another network must be reached by calling router B and having router B forward the data, that network is n hops away, where n is the number of routers traversed by the data to get to the network farthest away. When there are multiple paths to a network, the path with the fewest hops is chosen and is regarded as the best path, and all other information about how to get to that network is discarded.

Introduction 1-5