CCNA Study Guide Chapter 7 Summary

Chapter 7 began with a look at the basics of the boot process of a Cisco router. Next, the initial configuration of a Cisco router from the System Configuration Dialog prompts was explored. Extended setup mode was used to assign the router its initial configuration parameters including a hostname, passwords, and interface IP addresses.

Configuration from the IOS command line was looked at next, beginning with the process of logging into and out of a router. The differences between user and privileged EXEC mode was discussed, including how they can be identified by their associated command prompts.

A look at the configuration files on a Cisco router outlined the differences between the running configuration stored in RAM and the startup configuration stored in NVRAM. The syntax for the copy command was also looked at, including its use in saving the running configuration to the startup configuration. An overview of IOS shortcuts explained navigation techniques that can be used from the command line, as well as the use of truncated or shorthand commands. A look at the terminal history command explained how to change the buffer size, or disable it within a session.

The IOS help function was explored next, including the ways in which it can be used to find or complete commands.

A look at IOS configuration modes introduced global configuration mode, as well as the difference between the configure terminal, memory, and network options. Various levels of global configuration mode were also explored, including those associated with interface, line, and routing protocol configuration. The router prompts associated with each were also explained.

The configuration of passwords was also looked at, including the purpose and configuration of enable, enable secret, console, auxiliary, and virtual terminal passwords. The service password-encryption command was also discussed as a way of encrypting passwords not usually encrypted by default. The ability to configure a logon banner was also explored, using the banner motd command.

The interface configuration section outlined the procedure for setting IP addresses on interfaces, as well as the importance of the no shutdown command. An overview of configuring serial interfaces as DCE for lab environments was also discussed. The procedure for enabling and configuring IPX was also looked at, including the configuration of Ethernet encapsulation settings for IPX. The show interface command was discussed as a way to gain information about an interface, including its physical and data link characteristics.

A look at hostnames explained how to change the hostname on a Cisco router, configure a hosts table, and finally how to configure a router to use a DNS server for name resolution.

The copy command was then looked at in more detail. It explained the backup and restoration of configuration files and IOS images, both between memory areas and via a TFTP server. A look at telnet explained the basics of initiating, disconnecting, and switching between sessions. Configuration of the router via a web browser was also briefly explained. Diagnostic utilities were looked at next, including both ping and traceroute.
A look at the Cisco Discovery Protocol followed, including how it can be used to gain valuable information about neighboring Cisco devices.

Cisco Discovery Protocol (CDP)

Cisco Discovery Protocol (CDP) is a proprietary protocol developed by Cisco that provides a quick and easy way to find out about neighboring Cisco devices on your network. Enabled by default, CDP provides a variety of information on neighboring Cisco routers and switches. CDP messages use the Ethernet SNAP frame type.

To view information about neighboring Cisco devices, use the show cdp neighbors command.

cisco2501#show cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater
> Device ID Local Intrfce Holdtme Capability Platform Port ID
accra Eth 0 146 R 2500 Eth 0

The information provided shows that our Cisco 2501 has received CDP announcements from one other piece of equipment – in this case, another Cisco 2500 router. Notice the capability codes listed first. These provide you with an overview of the different types of equipment that can be found via CDP – R designates a router, S a switch, and so forth. These codes are then associated with the devices found, under the Capability header.
Obviously a variety of information is provided by the show CDP neighbors command, but what does it all mean? Each section is looked at below.

  • Device ID. The hostname of the neighboring device.
  • Local Interface. The interface on which this router received information about the neighboring device.
  • Holdtime. The amount of time the router will store this information before dropping it from memory, if additional CDP packets are not received.
  • Capability. The type of device that announced itself using CDP.
  • Platform. The hardware platform of the neighboring equipment.
  • Port ID. The port from which the CDP packet was sent on the neighboring device.

To obtain more detailed information on any CDP neighboring device, use the show cdp neighbor detail command.

cisco2501#show cdp neighbor detail
-------------------------
Device ID: accra
Entry address(es):
IP address: 192.168.1.45
Novell address: 101A.0060.5cc4.f41b
Platform: cisco 2500, Capabilities: Router
Interface: Ethernet0, Port ID (outgoing port): Ethernet0
Holdtime : 178 sec

Version :
Cisco Internetwork Operating System Software
IOS ™ 2500 Software (C2500-D-L), Version 12.0(5), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Tue 15-Jun-99 20:08 by phanguye

This command provides six primary pieces of information about neighboring devices including its hostname, logical addresses (CDP will provide one logical address per protocol), platform, connected interface, CDP holdtime, and IOS version. If you’re looking for a simple way to determine an IP or IPX address of a neighboring device, this command is also the way to go.

By default, CDP packets are sent out every 60 seconds. The default holdtime is set to 180 seconds – again, this is the length of time that the device will hold information provided by CDP if an update is not received.

To view the CDP holddown and timer values currently configured on your router, use the show cdp command.

cisco2501#show cdp
Global CDP information:
Sending CDP packets every 60 seconds
Sending a holdtime value of 180 seconds

To reconfigure either value, use the cdp timer and cdp holdtime commands, as shown below. We’ll set the holdtime on the router to 200 seconds, and the timer value to 70 seconds. Remember that in order to change just about any setting, you will need to be in global configuration mode.

cisco2501#config t
Enter configuration commands, one per line. End with CNTL/Z.
cisco2501(config)#cdp holdtime 200
cisco2501(config)#cdp timer 70

In some cases, you may not want CDP enabled on a given interface. To turn it off for a single interface, access the interface’s configuration and issue the no cdp enable command.

cisco2501(config)#int e0
cisco2501(config-if)#no cdp enable

In the example above, I disabled CDP, but only on interface Ethernet 0. If you change your mind and want to re-enable CDP on an interface, use the cdp enable command. To globally enable or disable CDP, use the cdp run or no cdp run commands respectively from global configuration mode.

cisco2501(config)#cdp run
cisco2501(config)#no cdp run

To view the CDP properties of all interfaces on your router, use the show cdp interface command.

cisco2501#show cdp interface
Ethernet0 is up, line protocol is up
Encapsulation ARPA
Sending CDP packets every 60 seconds
Holdtime is 180 seconds
Serial0 is down, line protocol is down
Encapsulation HDLC
Sending CDP packets every 60 seconds
Holdtime is 180 seconds
Serial1 is down, line protocol is down
Encapsulation HDLC
Sending CDP packets every 60 seconds
Holdtime is 180 seconds

The Traceroute Diagnostic Utility

Another useful utility is testing connectivity, especially in routed environments, is traceroute. While ping tests for basic connectivity with another host, traceroute will show you the path that a packet takes (in terms of crossing intermediate routers) between a source and destination. Since we haven’t set up routing yet, traceroute won’t provide us with much useful information. In a routed environment, traceroute provides valuable information because it helps to indicate at which point in a packet’s travels a failure is occurring. Issues might include an intermediate router being offline, or physical connection problems.

Traceroute works by sending groups of 3 UDP datagrams to the destination address specified, with varying time to live (TTL) values. For example, imagine there are three routers between our system and the destination host that we’re to determine the path to. Traceroute will send out 3 UDP datagrams with a TTL of one. When these hit the first router in the path, their TTL will be decremented by one, causing the packets to expire. ICMP “time exceeded” messages will be sent back to the source host. It will then send out another 3 UDP datagrams with a TTL of 2, which will exceed their TTL at the second router. This process continues until the destination host is reached. The cumulative information provided shows the path to the destination. If the process fails at any point, this indicates or suggests a problem area between the source and destination. Traceroute is an exceptionally simple and powerful troubleshooting tool in routed environments. To use it, simply enter traceroute followed by the destination IP address or hostname.

cisco2501#traceroute 192.168.1.209
Type escape sequence to abort.
Tracing the route to 192.168.1.209
1 192.168.1.209 4 msec 40 msec *

As I mentioned previously, traceroute doesn’t provide very much information on our network yet. Once some routing is configured, we’ll be able to see multiple hops in the path to a destination.

The Ping Diagnostic Utility

You are probably familiar with the ping utility from Windows or Linux. The version included with the Cisco IOS provides significantly enhanced functionality, and can be used to test connectivity for a variety of different protocols including IP, IPX, AppleTalk and more. To get a sense of the functions provided by ping, issue the ping command followed by the question mark.

cisco2501#ping ?
WORD Ping destination address or hostname
appletalk Appletalk echo
decnet DECnet echo
ip IP echo
ipx Novell/IPX echo
tag Tag encapsulated IP echo

Notice the range of protocols that ping can work with. In fact, the list can be even longer depending on the protocols supported by your IOS version. At the most basic level, ping sends out echo request messages and expects to receive back echo replies. It is important to be clear about the information that a ping provides. For example, if you can ping an IP host on a different network, it suggests that both hosts have TCP/IP correctly initialized and configured, and that routing between the networks is also configured correctly. In cases where you cannot ping a remote host, don’t jump to the conclusion that the remote host is unavailable or misconfigured – though it might be, the problem may also be a configuration issue with the source host, or potentially some routing-related (or physical connectivity) issue between the two. As a general rule, use the following steps to determine the source of connectivity issues between your PC and a remote system:

  1. Assuming that your IP address, subnet mask, and default gateway are correct, attempt to ping a host on a different subnet. If this fails, one possibility is that routing is not configured correctly.
  2. If pinging a remote host fails, attempt to ping your default gateway. If this fails, it may indicate that TCP/IP is not configured cor