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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.

Connect Devices (1.1.2)

In this section, you will see how accessing a network involves connecting hosts and infrastructure devices with IP addresses, subnet masks, and default gateways. This section will also introduce how to configure the initial settings of a switch.

Connect to a Network (

Network devices and end users typically connect to a network using a wired Ethernet or wireless connection. Refer to the sample reference topology in Figure 1-6. The LANs in the figure serve as an example of how users and network devices could connect to networks.

Figure 1-6

Figure 1-6 Sample LAN and WAN Connections

Home Office devices can connect as follows:

  • Laptops and tablets connect wirelessly to a home router.
  • A network printer connects using an Ethernet cable to the switch port on the home router.
  • The home router connects to the service provider cable modem using an Ethernet cable.
  • The cable modem connects to the Internet service provider (ISP) network.

The Branch site devices connect as follows:

  • Corporate resources (i.e., file servers and printers) connect to Layer 2 switches using Ethernet cables.
  • Desktop PCs and voice over IP (VoIP) phones connect to Layer 2 switches using Ethernet cables.
  • Laptops and smartphones connect wirelessly to wireless access points (WAPs).
  • The WAPs connect to switches using Ethernet cables.
  • Layer 2 switches connect to an Ethernet interface on the edge router using Ethernet cables. An edge router is a device that sits at the edge or boundary of a network and routes between that network and another, such as between a LAN and a WAN.
  • The edge router connects to a WAN service provider (SP).
  • The edge router also connects to an ISP for backup purposes.

The Central site devices connect as follows:

  • Desktop PCs and VoIP phones connect to Layer 2 switches using Ethernet cables.
  • Layer 2 switches connect redundantly to multilayer Layer 3 switches using Ethernet fiber-optic cables (orange connections).
  • Layer 3 multilayer switches connect to an Ethernet interface on the edge router using Ethernet cables.
  • The corporate website server is connected using an Ethernet cable to the edge router interface.
  • The edge router connects to a WAN SP.
  • The edge router also connects to an ISP for backup purposes.

In the Branch and Central LANs, hosts are connected either directly or indirectly (via WAPs) to the network infrastructure using a Layer 2 switch.

Default Gateways (

To enable network access, devices must be configured with IP address information to identify the appropriate:

  • IP address: Identifies a unique host on a local network
  • Subnet mask: Identifies with which network subnet the host can communicate
  • Default gateway: Identifies the router to send a packet to when the destination is not on the same local network subnet

When a host sends a packet to a device that is on the same IP network, the packet is simply forwarded out of the host interface to the destination device.

When a host sends a packet to a device on a different IP network, then the packet is forwarded to the default gateway, because a host device cannot communicate directly with devices outside of the local network. The default gateway is the destination that routes traffic from the local network to devices on remote networks. It is often used to connect a local network to the Internet.

The default gateway is usually the address of the interface on the router connected to the local network. The router maintains routing table entries of all connected networks as well as entries of remote networks, and determines the best path to reach those destinations.

For example, if PC1 sends a packet to the Web Server located at, it would discover that the Web Server is not on the local network and it, therefore, must send the packet to the Media Access Control (MAC) address of its default gateway. The packet protocol data unit (PDU) in Figure 1-7 identifies the source and destination IP and MAC addresses.

Figure 1-7

Figure 1-7 Getting the Pieces to the Correct Network

Document Network Addressing (

When designing a new network or mapping an existing network, document the network. At a minimum, the documentation should identify:

  • Device names
  • Interfaces used in the design
  • IP addresses and subnet masks
  • Default gateway addresses

This information is captured by creating two useful network documents:

  • Topology diagram: Provides a visual reference that indicates the physical connectivity and logical Layer 3 addressing. Often created using software, such as Microsoft Visio.
  • Addressing table: A table that captures device names, interfaces, IPv4 addresses, subnet masks, and default gateway addresses.

Figure 1-8 displays the sample topology diagram, while Table 1-2 provides a sample addressing table for the topology.

Figure 1-8

Figure 1-8 Documenting Network Addressing

Table 1-2 Addressing Table



IP Address

Subnet Mask

Default Gateway















Enable IP on a Host (

A host can be assigned its IP address information in one of two ways. A host can get a:

  • Statically Assigned IP Address: The host is manually assigned the correct IP address, subnet mask, and default gateway. The DNS server IP address can also be configured.
  • Dynamically Assigned IP Address: IP address information is provided by a server using the Dynamic Host Configuration Protocol (DHCP). The DHCP server provides a valid IP address, subnet mask, and default gateway for end devices. Other information may be provided by the server.

Figures 1-9 and 1-10 provide static and dynamic IPv4 address configuration examples.

Figure 1-9

Figure 1-9 Statically Assigning an IP Address

Figure 1-10

Figure 1-10 Dynamically Assigning an IP Address

Statically assigned addresses are commonly used to identify specific network resources, such as network servers and printers. They can also be used in smaller networks with few hosts. However, most host devices acquire their IPv4 address information by accessing a DHCP server. In large enterprises, dedicated DHCP servers providing services to many LANs are implemented. In a smaller branch or small office setting, DHCP services can be provided by a Cisco Catalyst switch or a Cisco ISR.

Device LEDs (

Host computers connect to a wired network using a network interface and RJ-45 Ethernet cable. Most network interfaces have one or two LED link indicators next to the interface. Typically, a green LED means a good connection while a blinking green LED indicates network activity.

If the link light is not on, then there may be a problem with either the network cable or the network itself. The switch port where the connection terminates would also have an LED indicator lit. If one or both ends are not lit, try a different network cable.

Similarly, network infrastructure devices commonly use multiple LED indicators to provide a quick status view. For example, a Cisco Catalyst 2960 switch has several status LEDs to help monitor system activity and performance. These LEDs are generally lit green when the switch is functioning normally and lit amber when there is a malfunction.

Cisco ISRs use various LED indicators to provide status information. The LEDs on the router help the network administrator conduct some basic troubleshooting. Each device has a unique set of LEDs. Consult the device-specific documentation for an accurate description of the LEDs.

The LEDs of the Cisco 1941 router shown in Figure 1-11 are explained in Table 1-3.

Figure 1-11

Figure 1-11 Cisco 1941 LEDs

Table 1-3 Description of the Cisco 1941 LEDs







GE0/0 and GE0/1

S (Speed)

1 blink + pause

Port operating at 10 Mb/s

2 blink + pause

Port operating at 100 Mb/s

3 blink + pause

Port operating at 1000 Mb/s

L (Link)


Link is active


Link is inactive





Port is active


Port is inactive





Port is active


Port is inactive

Console Access (

In a production environment, infrastructure devices are commonly accessed remotely using Secure Shell (SSH) or HyperText Transfer Protocol Secure (HTTPS). Console access is really only required when initially configuring a device, or if remote access fails.

Console access requires:

  • Console cable: RJ-45-to-DB-9 console cable
  • Terminal emulation software: Tera Term, PuTTY, HyperTerminal

The cable is connected between the serial port of the host and the console port on the device. Most computers and notebooks no longer include built-in serial ports. If the host does not have a serial port, the USB port can be used to establish a console connection. A special USB-to-RS-232 compatible serial port adapter is required when using the USB port.

The Cisco ISR G2 supports a USB serial console connection. To establish connectivity, a USB Type-A to USB Type-B (mini-B USB) is required, as well as an operating system device driver. This device driver is available from http://www.cisco.com. Although these routers have two console ports, only one console port can be active at a time. When a cable is plugged into the USB console port, the RJ-45 port becomes inactive. When the USB cable is removed from the USB port, the RJ-45 port becomes active.

Table 1-4 summarizes the console connection requirements, while Figure 1-12 displays the various ports and cables required.

Table 1-4 Console Connection Requirements

Port on Computer

Cable Required

Port on ISR

Terminal Emulation

Serial Port

RJ-45 to DB-9 Console Cable

RJ-45 Console Port

Tera Term PuTTY

USB Type-A Port

USB to RS-232 compatible serial port adapter

  • Adapter may require a software driver

RJ-45 to DB-9 Console Cable

USB Type-A to USB Type-B (Mini-B USB)

  • A device driver is required and available from Cisco.com

USB Type-B(Mini-B USB)

Figure 1-12

Figure 1-12 Ports and Cables

Enable IP on a Switch (

Network infrastructure devices require IP addresses to enable remote management. Using the device IP address, the network administrator can remotely connect to the device using Telnet, SSH, HTTP, or HTTPS.

A switch does not have a dedicated interface to which an IP address can be assigned. Instead, the IP address information is configured on a virtual interface called a switched virtual interface (SVI).

The steps to configure the basic settings on a switch are as follows:


Step 1. Name the device.

Step 2. Configure the SVI. This makes the switch accessible for network management.

Step 3. Enable the SVI.

Step 4. Configure the default gateway for the switch. Packets generated by the switch and destined for an address other than its management network segment will be forwarded to this address. This default gateway is used by the switch only for the packets it generates, not any hosts connected to the switch.

For example, the following commands would configure the management VLAN interface and default gateway of switch S1 shown in Figure 1-13.

Figure 1-13

Figure 1-13 Configuring the SVI of S1

S1(config)# interface vlan 1
S1(config-if)# ip address
S1(config-if)# no shutdown
%LINK-5-CHANGED: Interface Vlan1, changed state to up
S1(config-if)# exit
S1(config)# ip default-gateway

In the example, the switch SVI is configured and enabled with the IP address and a default gateway of the router located at Packets generated by the switch and destined for an address outside of the network segment will be forwarded to this address. In the example, the address is that of the G0/0 interface of R1.

6. Basic Settings on a Router (1.1.3) | Next Section Previous Section

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