Network Communications in Routed Environments

In order to appreciate the role of a router, we need to take a look at how communication happens between hosts on different networks in a routed environment. For the purpose of these illustrations, I’m going to assume that our network is running the most popular routed protocol suite, TCP/IP. We’ll begin with a look at a simple internetwork consisting of two networks, both connected to a single router. This will be followed by a look at a larger internetwork, where multiple networks are connected via many routers.

Routed Versus Routing Protocols

Before we take a look at a how a path is determined, it’s important to be able to differentiate between a routed and routing protocol. Although the names are similar, there is a big difference between the two. A routed protocol is a Network Layer protocol that is used to move traffic between networks. IP, IPX, and AppleTalk are all examples of routed protocols. Routed protocols allow a host on one network to communicate with a host on another, with routers forwarding traffic between the source and destination networks. They are characterized by logical addressing (such as an IP or IPX address) that not only identifies a source or destination host, but also the network (or subnet) on which they reside. In contrast, a protocol like NetBEUI does not use any logical addressing, and isn’t routable. Why is that? Because when a router comes across a NetBEUI packet, it has no way of determining where the destination host resides, since a NetBEUI packet does not include a logical destination address, only a name. The protocol cannot be routed; this means that communication between NetBEUI hosts is limited to occurring within a single non-routed network. Obviously that limits NetBEUI’s usefulness on a large internetwork.

Routing protocols serve a different purpose. Instead of being used to send data between source and destination hosts, a routing protocol is used by routers to exchange routing information with one another. For example, if we want our routers to dynamically “learn” about networks from one another, we configure them with a common routing protocol such as RIP or IGRP. Routers use routing protocols to exchange information about the networks they are aware of. In other words, routing protocols allow routers to “talk” to one another. This doesn’t mean that we need to configure routing protocols on every internetwork – there are other options, such as statically defining paths to destination networks on each router. If that sounds like a lot of work, you’re right. Once a network moves beyond a few routers, you will definitely need to consider adding one or more routing protocols. A variety of routing protocols exist beyond RIP and IGRP, including OSPF, EIGRP, AURP and others. The reason for choosing one over another will be influenced by a number of factors, including the size of a network, required performance, and the routed protocol(s) in use.

Introduction to Routing

You may recall from previous chapters that in order for systems in one broadcast domain to communicate with systems in another broadcast domain, a router must be involved. A router usually acts as the demarcation point between broadcast domains. Remember that a broadcast domain is a Layer 2 concept. Routing, on the other hand, happens at Layer 3 of the OSI model – the Network Layer. We’ll look at how these two layers interact in a routed environment shortly.

The act of routing is primarily concerned with moving traffic between networks. This can be as simple as having two networks directly connected via a single router. More commonly, we will need to be concerned with moving traffic between networks across a larger internetwork – in other words, there may be many routers between a source and destination network. In order to get traffic from one network to another, we will need to somehow make routers aware of where they should next forward traffic, in order for it to reach its final destination. This can be done in different ways, each with associated pros and cons. Ultimately, the way in which we choose to configure our routers will depend on our goals in a specific network environment.

At the most basic level, a router does no more than make decisions about getting to a destination network. In that way, routing isn’t much different than driving a car between your home and work – there may be many ways to get to work, and ultimately you have to choose a path. At any given intersection, you have to make a decision. Should you turn the car and take a different path, or should you carry on and take what you usually consider to be the best route? Many factors will likely impact your decision. For example, you might consider the most scenic route, the one with the highest speed limit, or simply may be trying to avoid roadwork. Regardless of the route you choose, you ultimately make a decision at each major intersection on your drive. In the same way, routers are like intersections on a network – at each point, a decision has to be made as to where to send data next. There may be many ways to get to a destination, but ultimately the goal of routing is to determine the best path. What constitutes the “best” path depends on how your network is configured, as you will see shortly.

Understanding and Configuring Routing

In Chapter 7, you learned the basics of configuring a Cisco router from the IOS command line. After completing important initial tasks like setting passwords and configuring interfaces, it’s time to get the router to start serving its real purpose – routing traffic between networks. In order to do that, you need to take a look at more than just the commands required to configure routing and associated protocols. More importantly, you need to begin by understanding how routing works on a conceptual level and defining what it is that you’re actually trying to accomplish.

For the purpose of both the CCNA exam, you’ll need to understand how routing works, the differences between routing protocols, and finally how routing is configured. The topics that I will cover in this chapter include:

  • An introduction to routing
  • The difference between routed and routing protocols
  • Understanding how network communication occurs in a routed environment
  • Static IP routing
  • Distance vector versus link state routing protocols
  • Dynamic IP routing with the Routing Information Protocol (RIP)
  • Dynamic IP routing with Interior Gateway Routing Protocol (IGRP)
  • Default routing
  • Routing IPX traffic
  • The IPX Routing Information Protocol (RIP) and Service Advertising Protocol (SAP)