Internet Control Message Protocol (ICMP) to verify
the hardware connection and the logical address of the network
layer. Figure is a table showing the various ICMP message
types. This is a very basic testing mechanism for network
connectivity. In Figure , the ping target 172.16.1.5
responded successfully to all five datagrams sent. The
exclamation points (!) indicate each successful echo. If
one or more periods (.) instead of exclamations are
received on a display, the application on the router timed out
waiting for a given packet echo from the ping target.
Command Purposes Router#ping [protocol]
{host | address} Invokes a diagnostic tool for testing
connectivity The ping command tests network connections
by sending ICMP echo requests to a target host and times
replies. When testing a connection, ping tracks the number of
packets sent, the number of replies received, and the
percentage of packets lost. It also tracks the amount of time
required for the packets to reach the destination, and for
replies to be received. This information allows verification of
communication between a workstation and other hosts, and
whether information was lost. The ping command can be
invoked from both user EXEC mode and privileged EXEC mode. The
ping command can be used to confirm basic network
connectivity on AppleTalk, ISO Connectionless Network Service
(CLNS), IP, Novell, Apollo, VINES, DECnet, or XNS networks. The
use of an extended ping command directs the router to
perform a more extensive range of test options. To use extended
ping, type ping at the command line, then press
the Enter key without entering an IP address. Prompts
will appear each time the Enter key is pressed, allowing the
specification of many more options than with standard
ping. It is a good idea to use the ping command
when the network is functioning properly to see how the command
works under normal conditions and so that there is something to
compare against when troubleshooting. Lab Activity
e-Lab Activity: Layer 3 Troubleshooting Using ping In
this lab, the students will use the ping command to
send ICMP echo requests to an adjacent router. Web
Links Using the Extended ping and Extended
traceroute Commands http://www.cisco.com/en/US/tech/ tk826/
tk365/technologies_ tech_ note09186a0080093f22.shtml
Content 9.2 Network Testing
9.2.6 Layer 7 troubleshooting using Telnet The
telnet utility is a virtual terminal protocol that is part of
the TCP/IP protocol suite. It allows verification of the
application layer software between source and destination
stations. This is the most complete test mechanism available.
The telnet utility is normally used to connect remote devices,
to gather information, and to run programs. The Telnet
application provides a virtual terminal for connection to
routers running TCP/IP. For troubleshooting purposes, it is
useful to verify that a connection can be made using Telnet.
This proves that at least one TCP/IP application is able to
connect end-to-end. A successful Telnet connection indicates
that the upper-layer application and the services of lower
layers are functioning properly. If an administrator can Telnet
to one router but not to another router, verify lower layer
connectivity. If connectivity has been verified, it is likely
that the Telnet failure is caused by specific addressing,
naming, or access permission problems. These problems can exist
on the administrator's router or on the router that failed as a
Telnet target. If the Telnet to a particular server fails from
one host, try connecting from a router and several other
devices. When trying to Telnet, if a login prompt is not
achieved, check the following: - Can a reverse DNS
lookup on the client's address be found? Many Telnet servers
will not allow connections from IP addresses that have no DNS
entry. This is a common problem for DHCP-assigned addresses in
which the administrator has not added DNS entries for the DHCP
pools.
- It is possible that a Telnet application cannot
negotiate the appropriate options and therefore will not
connect. On a Cisco router, this negotiation process can be
viewed using debug telnet.
- It is
possible that Telnet is disabled or has been moved to a port
other than 23 on the destination server.
Lab
Activity Lab Exercise: Troubleshooting Using Ping and
Telnet This lab is to gather information about routing updates
and routing protocols. Interactive Media Activity Point
and Click: Telnet After completing this activity, the student
will be able to understand Telnet. Web Links
telnet Command http://www.cisco.com/en/US/products/sw/
iosswrel/ps1835/ products_command_reference_
chapter09186a008008710e.html#998293
Content
9.3 Troubleshooting Router Issues
Overview 9.3.1 Troubleshooting Layer 1 using
show interfaces The Cisco IOS contains a rich set of
commands for troubleshooting. Among the more widely used are
the show commands. Every aspect of the router can be
viewed with one or more of the show commands. The
show command used to check the status and statistics of
the interfaces is the show interfaces command.
Variations of this command can be used to check the status of
the different types of interfaces. To view the status of the
FastEthernet interfaces, use show interfaces
FastEthernet. The command may also be used to view the
status of one particular interface. To view the status of
Serial 0/0, use show interfaces serial0/0.The status of
two important portions of the interfaces is shown with the
show interfaces command. They are the physical (hardware)
portion and logical (software) portion. These can be related to
the Layer 1 and the Layer 2 functions. The hardware includes
cables, connectors, and interfaces showing the condition of the
physical connection between the devices. The software status
shows the state of messages such as keepalives, control
information, and user information that are passed between
adjacent devices. This relates to the condition of a Layer 2
protocol passed between two connected router interfaces. These
important elements of the show interfaces serial command
output are displayed as the line and data-link protocol status.
The first parameter refers to the hardware layer and
essentially reflects whether the interface is receiving the
Carrier Detect (CD) signal from the other end of the
connection. If the line is down, a problem may exist with the
cabling, equipment somewhere in the circuit may be powered off
or malfunctioning, or one end may be administratively down. If
the interface is administratively down it has been manually
disabled in the configuration. The show interfaces serial
command also provides information to help diagnose other
Layer 1 issues that are not as easy to determine. An increasing
number of carrier transitions counts on a serial link may
indicate one or more of the following problems: - Line
interruptions due to problems in the service provider
network.
- Faulty switch, DSU, or router hardware.
If an increasing number of input errors appear in the
show interfaces serial output, there are several
possible sources of those errors. Some of these related to
Layer 1 problems are: - Faulty telephone company
equipment
- Noisy serial line
- Incorrect cable
or cable length
- Damaged cable or connection
- Defective CSU or DSU
- Defective router
hardware
Another area to examine is number of
interface resets. These are the result of too many missed
keepalives. The following Layer 1 problems could be a cause of
interface resets: - Bad line causing carrier
transitions
- Possible hardware problem at the CSU, DSU,
or switch
If carrier transitions and interface
resets are increasing or if input errors are high while
interface resets are increasing, the problem is likely to be a