106 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 9 Copyright © 2004, Cisco Systems, Inc.
9.2.3 Testing by OSI layers Testing should begin with Layer 1 of the OSI model and work to Layer 7 if necessary. The
ping command is used at Layer 3. The telnet command is used at Layer 7. Both of these
commands will be discussed in detail in a later section. It is important for the students to
understand which types of errors occur at the different layers of the OSI model. This is a good
opportunity to group students together to practice for an exam or earn extra credit through
competitive activities. For example, describe a type of error and have the teams compete to
determine the associated layer.
Relevant TIs from CCNA 2 v2.1.4 are 13.1.1 and 13.1.5.
The students need to understand the ping process and what is tested by each ping:
• ping the loopback address
• ping the interface address
• ping the local router interface address
• ping a remote host address
9.2.4 Layer 1 troubleshooting using indicators Indicator lights are useful troubleshooting tools. Most interfaces or NICs have indicator lights
that show if there is a valid connection. The interface may also have lights to indicate if traffic
is being sent or received. Have the students discuss possible Layer 1 problems. Instruct
students to check for the simplest of problems first such as power cords or electricity.
9.2.5 Layer 3 troubleshooting using ping The ping utility is used to test network connectivity. Echo protocols are used to test if protocol
packets are being routed. The ping command sends a packet to the destination host and then
waits for a reply packet from that host. Results from this echo protocol can help evaluate the
path-to-host reliability, delays over the path, and if the host can be reached or is functional.
The ping command can be invoked from both user EXEC mode and privileged EXEC mode.
To use the extended ping command, the user must be in privileged EXEC mode. Explain to
the students that it is a good idea to use the ping command when the network functions
properly to see how the command works under normal conditions. This can also be used to
make comparisons when troubleshooting. The students should relate ping to reachability.
9.2.6 Layer 7 troubleshooting using Telnet Telnet is a virtual terminal protocol that is part of the TCP/IP protocol suite. Telnet allows the
verification between source and destination stations. The telnet command provides a virtual
terminal so administrators can use Telnet operations to connect with other routers that use
TCP/IP. The telnet command will be discussed in later sections of the curriculum. At this
point, students should understand the functions of Telnet.