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Cisco Networking Academy's Introduction to Routing Concepts

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Mar 20, 2014.

Chapter Description

This chapter introduces and explains the primary functions and features of a router and explains the process for connecting and configuring devices to the router. It continues by describing the process by which routers manage packets, determine data paths, and build routing tables.

Initial Configuration of a Router (1.1)

A router is essentially a special-purpose computer with an internetwork operating system optimized for the purpose of routing and securing networks. This section will examine the functions of a router and how a router determines the best path. It will also review the command-line interface (CLI) commands required to configure the base settings of a router.

Characteristics of a Network (

Networks have had a significant impact on our lives. They have changed the way we live, work, and play.

Networks allow us to communicate, collaborate, and interact in ways we never did before. We use the network in a variety of ways, including web applications, IP telephony, video conferencing, interactive gaming, electronic commerce, education, and more.

There are many terms, key structures, and performance-related characteristics that are referred to when discussing networks. These include:

  • Topology: There are physical and logical topologies. The physical topology is the arrangement of the cables, network devices, and end systems. It describes how the network devices are actually interconnected with wires and cables. The logical topology is the path over which the data is transferred in a network. It describes how the network devices appear connected to network users.
  • Speed: Speed is a measure of the data rate in bits per second (b/s) of a given link in the network.
  • Cost: Cost indicates the general expense for purchasing of network components, and installation and maintenance of the network.
  • Security: Security indicates how protected the network is, including the information that is transmitted over the network. The subject of security is important, and techniques and practices are constantly evolving. Consider security whenever actions are taken that affect the network.
  • Availability: Availability is a measure of the probability that the network is available for use when it is required.
  • Scalability: Scalability indicates how easily the network can accommodate more users and data transmission requirements. If a network design is optimized to only meet current requirements, it can be very difficult and expensive to meet new needs when the network grows.
  • Reliability: Reliability indicates the dependability of the components that make up the network, such as the routers, switches, PCs, and servers. Reliability is often measured as a probability of failure or as the mean time between failures (MTBF).

These characteristics and attributes provide a means to compare different networking solutions.

Why Routing? (

How does clicking a link in a web browser return the desired information in mere seconds? Although there are many devices and technologies collaboratively working together to enable this, the primary device is the router. Stated simply, a router connects one network to another network.

Communication between networks would not be possible without a router determining the best path to the destination and forwarding traffic to the next router along that path. The router is responsible for the routing of traffic between networks.

When a packet arrives on a router interface, the router uses its routing table to determine how to reach the destination network. The destination of the IP packet might be a web server in another country or an email server on the local-area network. It is the responsibility of routers to deliver those packets efficiently. The effectiveness of internetwork communications depends, to a large degree, on the ability of routers to forward packets in the most efficient way possible.

Routers Are Computers (

Most network capable devices (i.e., computers, tablets, and smartphones) require the following components to operate:

  • Central processing unit (CPU)
  • Operating system (OS)
  • Memory and storage (RAM, ROM, NVRAM, Flash, hard drive)

A router is essentially a specialized computer. It requires a CPU and memory to temporarily and permanently store data to execute operating system instructions, such as system initialization, routing functions, and switching functions.

Routers store data using:

  • Random Access Memory (RAM): Provides temporary storage for various applications and processes, including the running IOS, the running configuration file, various tables (i.e., IP routing table, Ethernet ARP table), and buffers for packet processing. RAM is referred to as volatile because it loses its contents when power is turned off.
  • Read-Only Memory (ROM): Provides permanent storage for bootup instructions, basic diagnostic software, and a limited IOS in case the router cannot load the full featured IOS. ROM is firmware and referred to as non-volatile because it does not lose its contents when power is turned off.
  • Non-Volatile Random Access Memory (NVRAM): Provides permanent storage for the startup configuration file (startup-config). NVRAM is non-volatile and does not lose its contents when power is turned off.
  • Flash: Provides permanent storage for the IOS and other system-related files. The IOS is copied from flash into RAM during the bootup process. Flash is non-volatile and does not lose its contents when power is turned off.

Table 1-1 provides a summary of the types of router memory, their volatility, and examples of what is stored in each.

Table 1-1 Router Memory






  • Running IOS
  • Running configuration file
  • IP routing and ARP tables
  • Packet buffer



  • Bootup instructions
  • Basic diagnostic software
  • Limited IOS



  • Startup configuration file



  • IOS file
  • Other system files

Unlike a computer, a router does not have video adapters or sound card adapters. Instead, routers have specialized ports and network interface cards to interconnect devices to other networks. Figure 1-1 displays the back panel of a Cisco 1941 ISRG2 and identifies those special ports and interfaces.

Figure 1-1

Figure 1-1 Back Panel of a 1941 ISRG2

Routers Interconnect Networks (

Most users are unaware of the presence of numerous routers on their own network or on the Internet. Users expect to be able to access web pages, send emails, and download music, regardless of whether the server accessed is on their own network or on another network. Networking professionals know that it is the router that is responsible for forwarding packets from network to network, from the original source to the final destination.

A router connects multiple networks, which means that it has multiple interfaces that each belong to a different IP network. When a router receives an IP packet on one interface, it determines which interface to use to forward the packet to the destination. The interface that the router uses to forward the packet may be the final destination, or it may be a network connected to another router that is used to reach the destination network.

Each network that a router connects to typically requires a separate interface. These interfaces are used to connect a combination of both local-area networks (LANs) and wide-area networks (WANs). LANs are commonly Ethernet networks that contain devices, such as PCs, printers, and servers. WANs are used to connect networks over a large geographical area. For example, a WAN connection is commonly used to connect a LAN to the Internet service provider (ISP) network.

Notice that each site in Figure 1-2 requires the use of a router to interconnect to other sites. Even the Home Office requires a router. In this topology, the router located at the Home Office is a specialized device that performs multiple services for the home network.

Figure 1-2

Figure 1-2 Sample Routed Topology

Routers Choose Best Paths (

The primary functions of a router are to:

  • Determine the best path to send packets
  • Forward packets toward their destination

The router uses its routing table to determine the best path to use to forward a packet. When the router receives a packet, it examines the destination address of the packet and uses the routing table to search for the best path to that network. The routing table also includes the interface to be used to forward packets for each known network. When a match is found, the router encapsulates the packet into the data link frame of the outgoing or exit interface, and the packet is forwarded toward its destination.

It is possible for a router to receive a packet that is encapsulated in one type of data link frame, and to forward the packet out of an interface that uses a different type of data link frame. For example, a router may receive a packet on an Ethernet interface, but must forward the packet out of an interface configured with the Point-to-Point Protocol (PPP). The data link encapsulation depends on the type of interface on the router and the type of medium to which it connects. The different data link technologies that a router can connect to include Ethernet, PPP, Frame Relay, DSL, cable, and wireless (802.11, Bluetooth).

Packet Forwarding Mechanisms (

Routers support three packet-forwarding mechanisms:

  • Process switching: An older packet-forwarding mechanism still available for Cisco routers. When a packet arrives on an interface, it is forwarded to the control plane, where the CPU matches the destination address with an entry in its routing table, and then determines the exit interface and forwards the packet. It is important to understand that the router does this for every packet, even if the destination is the same for a stream of packets. This process-switching mechanism is very slow and rarely implemented in modern networks. Figure 1-3 illustrates how packets are process-switched.

    Figure 1-3

    Figure 1-3 Process Switching

  • Fast switching: This is a common packet-forwarding mechanism which uses a fast-switching cache to store next-hop information. When a packet arrives on an interface, it is forwarded to the control plane, where the CPU searches for a match in the fast-switching cache. If it is not there, it is process-switched and forwarded to the exit interface. The flow information for the packet is also stored in the fast-switching cache. If another packet going to the same destination arrives on an interface, the next-hop information in the cache is re-used without CPU intervention. Figure 1-4 illustrates how packets are fast-switched.

    Figure 1-4

    Figure 1-4 Fast Switching

  • Cisco Express Forwarding (CEF): CEF is the most recent and preferred Cisco IOS packet-forwarding mechanism. Like fast switching, CEF builds a Forwarding Information Base (FIB) and an adjacency table. However, the table entries are not packet-triggered like fast switching but change-triggered such as when something changes in the network topology. Therefore, when a network has converged, the FIB and adjacency tables contain all the information a router would have to consider when forwarding a packet. The FIB contains pre-computed reverse lookups and next-hop information for routes, including the interface and Layer 2 information. Cisco Express Forwarding is the fastest forwarding mechanism and the preferred choice on Cisco routers. Figure 1-5 illustrates how packets are forwarded using CEF.

    Figure 1-5

    Figure 1-5 Cisco Express Forwarding

Figures 1-3 to 1-5 illustrate the differences between the three packet-forwarding mechanisms. Assume a traffic flow consisting of five packets all going to the same destination. As shown in Figure 1-3, with process switching, each packet must be processed by the CPU individually. Contrast this with fast switching, as shown in Figure 1-4. With fast switching, notice how only the first packet of a flow is process-switched and added to the fast-switching cache. The next four packets are quickly processed based on the information in the fast-switching cache. Finally, in Figure 1-5, CEF builds the FIB and adjacency tables, after the network has converged. All five packets are quickly processed in the data plane.

A common analogy used to describe the three packet-forwarding mechanisms is as follows:

  • Process switching solves a problem by doing math long hand, even if it is the identical problem.
  • Fast switching solves a problem by doing math long hand one time and remembering the answer for subsequent identical problems.
  • CEF solves every possible problem ahead of time in a spreadsheet.
5. Connect Devices (1.1.2) | Next Section Previous Section

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