Module
Overview
20.1 Examining
the Routing Table
20.1.1 The show
ip route command
20.1.2
Determining the gateway of last resort
20.1.3
Determining route source and destination
20.1.4 Determining
L2 and L3 addresses
20.1.5
Determining the route administrative distance
20.1.6
Determining the route metric
20.1.7
Determining the route next hop
20.1.8
Determining the last routing update
20.1.9 Observing
multiple paths to destination
20.2 Network
Testing
20.2.1
Introduction to network testing
20.2.2 Using a
structured approach to troubleshooting
20.2.3 Testing by
OSI layers
20.2.4 Layer 1
troubleshooting using indicators
20.2.5 Layer 3
troubleshooting using ping
20.2.6 Layer 7 troubleshooting
using Telnet
20.3
Troubleshooting Router Issues Overview
20.3.1
Troubleshooting Layer 1 using show interfaces
20.3.2
Troubleshooting Layer 2 using show interfaces
20.3.3
Troubleshooting using show cdp
20.3.4
Troubleshooting using traceroute
20.3.5
Troubleshooting routing issues
20.3.6
Troubleshooting using show controllers
20.3.7
Introduction to debug
Module:
Summary
Overview
A router uses a
dynamic routing protocol to learn about routes to destination networks. Most
routers use a combination of dynamic routing and manually configured static
routes. Regardless of the method used, when a router determines that a route is
the best path to a destination, it installs that route in its routing table.
This module will describe methods that are used to examine and interpret the
contents of the routing table.
Network testing
and troubleshooting are perhaps the most time consuming components of every
network administrator’s job. Efficient testing and troubleshooting must be done
in a logical, orderly, and well-documented fashion. Otherwise, the same
problems will reoccur, and the network administrator will never truly
understand the network. This module describes a structured approach to network
troubleshooting and provides some tools to use in the troubleshooting process.
Routing problems
are among the most common and difficult for network administrators to diagnose.
There are many tools that make it easier to identify and solve routing
problems. This module will introduce several of the most important of these
tools and provide practice in their use.
This module
covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND
640-811 exams. -
Students who
complete this module should be able to perform the following tasks:
- Use the show ip route command
to gather detailed information about the routes installed on the router
- Configure a default route or
default network
- Understand how a router uses
both Layer 2 and Layer 3 addressing to move data through the network
- Use the ping command to perform
basic network connectivity tests
- Use the telnet command to
verify the application layer software between source and destination
stations
- Troubleshoot by sequential
testing of OSI layers
- Use the show interfaces command
to confirm Layer 1 and Layer 2 problems
- Use the show ip route and show
ip protocol commands to identify routing issues
- Use the show cdp command to
verify Layer 2 connectivity
- Use the traceroute command to
identify the path packets take between networks
- Use the show controllers serial
command to ensure the proper cable is attached
- Use basicdebugcommands to
monitor router activity
20.1 Examining the Routing Table
20.1.1 The show ip route command
This page will
explain the functions of the show ip route command.
One of the
primary functions of a router is to determine the best path to a given
destination. A router learns paths, which are also called routes, from the
configurations entered by an administrator or from other routers through
routing protocols. Routers store this routing information in routing tables
using on-board random access memory (RAM). A routing table contains a list of
the best available routes. Routers use the routing table to make packet
forwarding decisions.
The show ip route
command displays the contents of the IP routing table. This table contains
entries for all known networks and subnetworks, as well as a code that
indicates how that information was learned. The following are some additional
commands that can be used with the show ip route command:
- show ip route connected
- show ip routeaddress
- show ip route rip
- show ip route igrp
- show ip route static
A routing table
maps network prefixes to an outbound interface.
When RTA receives a packet destined for 192.168.4.46, it looks for the
prefix 192.168.4.0/24 in its table. RTA then forwards the packet out interface
Ethernet0 based on the routing table entry. If RTA receives a packet destined
for 10.3.21.5, it sends that packet out Serial 0.
The example
routing table shows four routes for directly connected networks. These routes
are labeled with a C. RTA drops any packet destined for a network that is not
listed in the routing table. The routing table for RTA will have to include
more routes before it can forward to other destinations. There are two ways to
add new routes:
- Static routing
- An administrator manually defines routes to one or more destination
networks.
- Dynamic routing
- Routers follow rules defined by a routing protocol to exchange routing
information and independently select the best path.
Administratively
defined routes are said to be static because they do not change until a network
administrator manually programs the changes. Routes learned from other routers
are dynamic because they change automatically as directly connected routers
update each other with new information. Each method has fundamental advantages
and disadvantages.
The Lab Activity
will allow students to use the show ip route command to examine routing tables.
The next page
will define a gateway of last resort.
20.1
Examining the Routing Table
20.1.2 Determining the gateway of last resort
This page will
teach students how to configure a gateway of last resort and why it is used.
It is not
feasible, or even desirable, for a router to maintain routes to every possible
destination. Instead, routers keep a default route, or a gateway of last
resort. Default routes are used when the router is unable to match a
destination network with a more specific entry in the routing table. The router
uses this default route to reach the gateway of last resort in an effort to
forward the packet.
A key scalability
feature is that default routes keep routing tables as lean as possible. They
make it possible for routers to forward packets destined to any Internet host
without having to maintain a table entry for every Internet network. Default
routes can be statically entered by an administrator or dynamically learned
using a routing protocol.
Default routing
begins with the administrator. Before routers can dynamically exchange
information, an administrator must configure at least one router with a default
route. Depending on the desired results, an administrator can use either of the
following commands to statically configure a default route:
ip
default-network
or
ip route 0.0.0.0
0.0.0.0
The ip
default-network command is used to establish a default route in networks that
use dynamic routing protocols. The ip
default-network command is classful, which means if the router has a route to
the subnet indicated by this command, it installs the route to the major net.
The ip default-network command must be issued using the major net, in order to
flag the candidate default route.
The global
command ip default-network 192.168.17.0 defines the Class C network
192.168.17.0 as the destination path for packets that have no routing table
entries. Any routes to a network configured with ip default-network will be
flagged as a candidate for the default route.
The ip route
0.0.0.0/0 command can also be used to configure a default route.
Router(config)#ip
route prefix mask {address 1 interface } [distance ]
After configuring
a default route or default network, the command show ip route will show the
following:
Gateway of last
resort is 172.16.1.2 to network 0.0.0.0
The Lab Activity
will show students how to configure a gateway of last resort.
The next page
explains how path determination occurs.
20.1
Examining the Routing Table
20.1.3 Determining route source and destination
This page will explain how a
router chooses a path for packet delivery.
For traffic going
through a network cloud, path determination occurs at the network layer. The
path determination function enables a router to evaluate the available paths to
a destination and to establish the preferred handling of a packet. Routing
services use network topology information to evaluate network paths. This
information can be configured by the network administrator or collected through
dynamic processes that are used in the network.
The network layer
provides best-effort, end-to-end, packet delivery across interconnected
networks. The network layer uses the IP routing table to send packets from the
source network to the destination network. After the router determines which
path to use, it forwards the packet from one interface to the interface or port
that leads to the destination.
The next page
will discuss Layer 2 and Layer 3 addresses.
20.1
Examining the Routing Table
20.1.4 Determining L2 and L3 addresses
This page will
explain how Layer 2 and Layer 3 addresses are used to route a packet.
For a packet to
get from the source to the destination, both Layer 2 and Layer 3 addresses are
used. Figure explains the process that
occurs as a packet moves through a network.
The Layer 3
address is used to route the packet from the source network to the destination
network. The source and destination IP addresses remain the same. The MAC
address changes at each hop or router. A data link layer address is necessary
because delivery within the network is determined by the address in the Layer 2
frame header.
The Interactive
Media Activity will help students recognize physical and logical addresses.
The next page
will describe the administrative distance of a route.
20.1
Examining the Routing Table
20.1.5 Determining the route administrative
distance
This page will
teach students what the administrative distance of a route is and how it is
used.
A router can
discover routes through dynamic routing protocols or routes can be configured
manually. After the routes are discovered or configured, the router must choose
the best routes to other networks.
The router uses
the administrative distance of each route to determine the best path to a particular
destination. The administrative distance is a number that measures the
trustworthiness of the source of the route information. The lower the
administrative distance, the more trustworthy the source.
Different routing
protocols have different default administrative distances. The path with the lowest administrative
distance is installed in the routing table.
In the Lab
Activity, students will analyze two routing protocols.
The next page
will discuss route metrics.
20.1
Examining the Routing Table
20.1.6 Determining the route metric
This page will
explain what route metrics are and how they are used. Students will also learn
how IGRP calculates route metrics.
Routing protocols use metrics
to determine the best route to a destination. The metric is a value that
measures the desirability of a route. Some routing protocols use only one
factor to calculate a metric. For example, RIP v1 uses hop count as the only
factor to determine the metric of a route. Other protocols base their metric on
hop count, bandwidth, delay, load, reliability, and cost.
Each routing
algorithm interprets what is best in its own way. The algorithm generates a
number, called the metric value, for each path through the network. A lower
metric number generally indicates a better path.
Factors such as bandwidth and
delay are static because they remain the same for each interface until the
router is reconfigured or the network is redesigned. Factors such as load and
reliability are dynamic because they are calculated for each interface in
real-time by the router.
The more factors
that make up a metric, the greater the flexibility to tailor network operations
to meet specific needs. By default, IGRP uses the static factors bandwidth and
delay to calculate a metric value. These two factors can be configured manually
to control which routes a router chooses. IGRP may also be configured to
include the dynamic factors of load and reliability in the metric calculation.
By using dynamic factors, IGRP routers can make decisions based on current
conditions. If a link becomes heavily loaded or unreliable, IGRP will increase
the metric of routes using that link. An alternate route with a lower metric
would be used instead.
IGRP calculates
the metric by adding the weighted values of different characteristics of the
link to the network in question. Here is the formula for calculating the
composite metric for IGRP:
Metric = [K1 *
Bandwidth + (K2 * Bandwidth)/(256-load) + K3*Delay] * [K5/(reliability + K4)]
The default constant
values are K1 = K3 = 1 and K2 = K4 = K5 = 0.
If K5 = 0, the
[K5/(reliability + K4)] term is not used. Given the default values for K1
through K5, the composite metric calculation used by IGRP reduces to Metric =
Bandwidth + Delay.
The Interactive
Media Activity will help students understand route metrics.
The next page
explains how a next hop is chosen.
20.1
Examining the Routing Table
20.1.7 Determining the route next hop
This page will explain how a
router determines the next hop for a packet.
Routing
algorithms fill routing tables with a variety of information. Destination next
hop associations determine the best path and which router to forward the packet
to next. This router represents the next hop on the way to the final destination.
When a router
receives an incoming packet, it checks the destination address and attempts to
associate this address with a next hop.
The next page
shows students how to find the last routing update.
20.1
Examining the Routing Table
20.1.8
Determining the last routing update
This page lists
some commands that are used to find the last routing update:
- show ip route
- show ip route address
- show ip protocols
- show ip rip database
The Lab Activity
will help students collect information about routing updates.
The next page
will discuss multiple paths to the same destination.
20.1 Examining the Routing Table
20.1.9 Observing multiple paths to destination
This page will discuss the
support of multiple paths to the same destination by some routing protocols.
Multi-path
algorithms permit traffic over multiple lines, provide better throughput, and
are more reliable than single path algorithms.
IGRP supports
unequal cost path load balancing, which is known as variance. The variance
command instructs the router to include routes with a metric less than n times
the minimum metric route for that destination, where n is the number specified
by the variance command. The variable n can take a value between 1 and 128,
with the default being 1, which means equal cost load balancing.
rt1 has two
routes to network 192.168.30.0. The variance command will be set on rt1 to
ensure that both paths to network 192.168.30.0 are utilized.
Figure shows the output from show ip route from rt1
before the variance is configured. FastEthernet 0/0 is the only route to
192.168.30.0. This route has an Administrative Distance of 100 and a metric of
8986.
Figure shows the output from show ip route from rt1
after the variance is configured. The preferred route is interface FastEthernet
0/0, but Serial 0/0 will also be used. After the variance command is executed,
IGRP will use load balancing between the two links.
This page
concludes this lesson. The next lesson will discuss network testing. The lesson
begins with an overview.
20.2 Network Testing
20.2.1 Introduction to network testing
This page will
give students an overview of how to test a network.
Basic testing of
a network should proceed in sequence from one OSI reference model layer to the
next. Begin with Layer 1 and work up to
Layer 7, if necessary. At Layer 1, look for simple problems such as power cords
plugged in the wall and other physical connections. The most common problems that
occur on IP networks result from errors in the addressing scheme. It is
important to test the address configuration before continuing with further
configuration steps.
Each test
presented in this lesson focuses on network operations at a specific layer of
the OSI model. At Layer 3, the commands telnet and ping are used to test the
network.
The next page
will discuss the troubleshooting process.
20.2 Network Testing
20.2.2 Using a structured approach to
troubleshooting
Troubleshooting
is a process that allows a user to find problems on a network. This page
explains why an orderly process should be used to troubleshoot a network. This
process should be based on the networking standards set in place by a network
administrator. Documentation is a very important part of the troubleshooting
process.
The steps in this
model are as follows:
Step 1 Collect
all available information and analyze the symptoms of the failure.
Step 2 Localize
the problem to a particular network segment, module, unit, or user.
Step 3 Isolate
the trouble to specific hardware or software within the unit, module, or user
network account.
Step 4 Locate and
correct the problem.
Step 5 Verify
that the problem has been solved.
Step 6 Document
the problem and the solution.
Figure shows another approach to troubleshooting.
These are not the only ways to troubleshoot a network. However, an orderly
process is important to keep a network running smoothly and efficiently.
When a structured
approach is used, every member of a network support team knows which steps the
other team members have completed to troubleshoot the network. If a variety of
troubleshooting ideas are tried with no organization or documentation, problem
solving is not efficient. Even if a problem is solved in the non-structured
environment, it will be difficult to replicate the solution for similar
problems.
The Interactive
Media Activity will help students become familiar with the troubleshooting
process.
The next page
will teach students the types of errors that occur at the first three layers of
the OSI model.
20.2 Network Testing
20.2.3 Testing by OSI layers
This page will describe the
types of errors that occur at the first three layers of the OSI model.
Layer 1 errors
can include the following:
- Broken cables
- Disconnected cables
- Cables connected to the wrong
ports
- Intermittent cable connection
- Rollover, crossover, or
straight-through cables used incorrectly
- Transceiver problems
- DCE cable problems
- DTE cable problems
- Devices turned off
Layer 2 errors
can include the following:
- Improperly configured serial
interfaces
- Improperly configured Ethernet
interfaces
- Improper encapsulation set
- Improper clockrate settings on
serial interfaces
- Network interface card (NIC)
problems
Layer 3 errors
can include the following:
- Routing protocol not enabled
- Wrong routing protocol enabled
- Incorrect IP addresses
- Incorrect subnet masks
If errors appear
on the network, the process of testing through the OSI layers should begin. The
ping command is used at Layer 3 to test connectivity. At Layer 7 the telnet
command may be used to verify the application layer software between source and
destination stations. Both of these commands will be discussed in detail in a
later section.
The Interactive
Media Activity will help students understand the types of problems that occur
at each layer of the OSI model.
The next page
will explain how indicator lights can be used to test a network.
20.2 Network Testing
20.2.4 Layer 1 troubleshooting using indicators
The page will
explain how to troubleshoot Layer 1 issues with the help of indicator lights.
Most interfaces or NICs have indicator lights that show if there is a valid
connection. This light is often called the link light. The interface may also
have lights to indicate when traffic is transmitted (TX) or received (RX). If
the interface has indicator lights that do not show a valid connection, check
for faulty or incorrect cabling. If cabling is correct, power off the device
and reseat the interface card.
Check to make
sure that all cables are connected to the appropriate ports. Make sure that all
cross-connects are properly patched to the correct location using the
appropriate cable and method.
Verify that the
proper cable is used. A crossover cable may be required for direct connections
between two switches or hubs, or between two hosts such as PCs or routers.
Verify that the cable from the source interface is properly connected and is in
good condition. If there is doubt that the connection is good, reseat the cable
and ensure that the connection is secure. Try replacing the cable with a known
working cable. If this cable connects to a wall jack, use a cable tester to
ensure that the jack is properly wired.
Also check any
transceiver in use to ensure that it is the correct type, is properly
connected, and is properly configured. If the problem continues after the cable
is replaced, replace the transceiver if one is used.
Always check to
make sure that the device is powered on. Always check the basics before running
diagnostics or attempting complex troubleshooting.
The next page
will describe the ping command.
20.2 Network Testing
20.2.5 Layer 3 troubleshooting using ping
This page will
explain how the ping utility can be used to test network connectivity. Many
network protocols support an echo protocol to help diagnose basic network
connectivity. Echo protocols are used to determine if protocol packets are
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 whether the
host can be reached or is functioning. The ping output displays the minimum,
average, and maximum times it takes for a ping packet to find a specified
system and return. The ping command uses ICMP to verify the hardware connection
and the logical address of the network layer. This is a very basic way to test
network connectivity. Figure shows the
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. Each
exclamation point (!) indicates a successful echo. One or more periods (.)
indicates that the application on the router timed out before it received a
packet echo from the ping target.
The following
command activates a diagnostic tool that is used to test connectivity:
Router#ping
[protocol] {host | address}
To test network
connectivity, the ping command sends ICMP echo requests to a target host and
measures how long it takes to reply. The ping command 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 packets to reach the
destination and for replies to be received. This information can be used to
verify communications between hosts and determine if 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, and press
the Enter key. Prompts will appear each time the Enter key is pressed. These
prompts provide many more options than with a standard ping.
Use the ping
command when the network functions properly to see how the command works under
normal conditions. This can be used as a comparison, or baseline, when
troubleshooting.
The Lab Activity
will allow students to use the ping command to send an ICMP echo request.
The next page
will describe the Telnet application.
20.2
Network Testing
20.2.6 Layer 7 troubleshooting using Telnet
This page will explain how
Telnet can be used to troubleshoot the application layer.
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 connection to routers that use 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, Telnet from a router and other
devices. If a login prompt is not achieved during Telnet, check the following:
- A reverse DNS lookup may not be
found on the client address. Many Telnet servers will not allow
connections from IP addresses that have no DNS entry. This is a common
problem for DHCP-assigned addresses if the administrator has not added DNS
entries for the DHCP pools.
- It is possible that a Telnet application
cannot negotiate the appropriate options and will not connect. On a Cisco
router, this negotiation process can be viewed with the debug telnet
command.
- It is possible that Telnet is
disabled or has been moved to a port other than 23 on the destination
server.
The Lab Activity
will allow students to troubleshoot a network with Telnet and the ping command.
The Interactive Media Activity will help students become more familiar with
Telnet.
This page
concludes this lesson. The next lesson will teach students how to troubleshoot
router issues. The first page will discuss show commands.
20.3 Troubleshooting Router Issues Overview
20.3.1 Troubleshooting Layer 1 using show
interfaces
This page will
discuss show commands and explain how they are used to troubleshoot Layer 1
issues.
The Cisco IOS
contains many 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. The show interfaces command without
arguments returns status and statistics on all the router ports. The show
interfaces <interface name> returns the status and statistics of only the
named port. To view the status of Serial 0/0, use show interfaces serial 0/0.
The status of two
important portions of the interfaces is shown with the show interfaces command.
They are the physical, or hardware portion and logical, or 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 can be demonstrated by an example of a serial port on a modular
router. The show interfaces serial 0/0 command displays the line and data-link
protocol status of serial port one.
The first
parameter refers to the hardware layer and indicates if the interface receives
a 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 0/0 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 0/0 output, there
are several possible sources of those errors. Some common Layer 1 problems are
as follows:
- 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 that causes 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 this occurs, the problem is likely to be a bad link or defective CSU or
DSU.
The number of
errors should be interpreted relative to the amount of traffic that the router
has processed and the amount of time that the statistics have been captured.
The router tracks statistics that provide information about the interface. The
statistics reflect router operation since it was started or since the last time
the counters were cleared.
If the show
interfaces output shows the last clearing of the counters as never, use the
show version command to find out how long the router has been functional.
Use the clear
counters privileged EXEC command to reset the counters to zero. These counters
should always be cleared after an interface problem has been corrected. This
reset to zero gives a better picture of the current status of the network and
will help verify that an issue has been corrected.
The Lab Activity
will help students become more familiar with the show interfaces command.
The next page
will explain how the show interfaces command is used to troubleshoot Layer 2
problems.
20.3
Troubleshooting Router Issues Overview
20.3.2 Troubleshooting Layer 2 using show
interfaces
This page will
further explain why the show interfaces command may be the most important tool
to discover Layer 1 and Layer 2 problems with the router. The first parameter,
which is line, refers to the physical layer. The second parameter, which is
protocol, indicates if the IOS processes that control the line protocol
consider the interface usable. This is determined by whether keepalives are
successfully received. Keepalives are defined as messages sent by one network
device to inform another network device that the virtual circuit between the
two is still active. If the interface misses three consecutive keepalives, the
line protocol is marked as down.
When the line is
down, the protocol is always down, because there is no useable media for the
Layer 2 protocol. This will be true when the interface is down due to a
hardware problem and when it is administratively down.
If the interface
is up and the line protocol is down, a Layer 2 problem exists. Possible causes
are as follows:
- No keepalives
- No clock rate
- Mismatch in encapsulation type
The show
interfaces command should be used after configuring a serial interface to
verify the changes and that the interface is operational.
The Interactive
Media activity will help students understand the show interfaces command.
The next page
will discuss the show cdp command.
20.3 Troubleshooting Router Issues Overview
20.3.3 Troubleshooting using show cdp
This page will
explain the functions of the show cdp command.
CDP advertises
device information to its direct neighbors. This includes MAC and IP addresses
and outgoing interfaces.
The output from
the show cdp neighbors command displays information about directly connected
Cisco device neighbors. This information
is useful for debugging connectivity issues. If a cabling problem is suspected,
enable the interfaces with the no shutdown command and then execute the show
cdp neighbors detail command before any other configuration. The command displays
specific device detail such as the active interfaces, the port ID, and the
device. The version of Cisco IOS that is running on the remote devices is also
shown.
If the physical layer functions
properly, then all other directly connected Cisco devices should be displayed.
If no known device appears, this usually indicates a Layer 1 problem.
One area of
concern with CDP is security. The amount of information CDP provides is so
extensive that it can be a potential security hole. For security reasons CDP should be configured
only on links between Cisco devices and disabled on user ports or links that
are not locally managed.
In the Lab
Activity, students will use the show cdp command to learn about network
devices.
The next page
will discuss the traceroute command.
20.3 Troubleshooting Router Issues Overview
20.3.4 Troubleshooting using traceroute
This page will
describe the functions of the traceroute command.
The traceroute
command is often referred to as the trace command in reference materials.
However, the correct command syntax is traceroute. The traceroute command
displays the routes that packets take to their destinations. The traceroute
command can also be used to test each hop at the network layer and provide
performance benchmarks.
The output of the
traceroute command generates a list of hops that were successfully
reached. If the data successfully
reaches the intended destination, then the output indicates every router that
the datagram passes through. This output can be captured and used for future
troubleshooting of the internetwork.
Traceroute output will also
indicate the specific hop at which the failure occurs. A line of output is
generated on the terminal for each router in the path. This indicates the IP
address of the interface that the data entered. If an asterisk (*) appears, the
packet failed. To isolate the problem area, obtain the last good hop from the
traceroute output and compare it to a diagram of the internetwork.
The traceroute
command also provides information about the performance of links. The round
trip time (RTT) is the time required to send a packet and get a response. This is useful for an approximate idea of the
delay on the link. These figures are not precise enough to be used for an
accurate performance evaluation. However, this output can be captured and used
for future performance troubleshooting of the internetwork.
Notice that the
device receiving the traceroute also has to know how to send the reply back to
the source of the traceroute. For the traceroute or ping data to make the round
trip between routers, there must be known routes in both directions. A failed
response is not always an indication of a problem because ICMP messages could
be rate-limited or filtered at the host site. This is especially true across
the Internet.
Traceroute sends
out a sequence of User Datagram Protocol (UDP) datagrams from the router to an
invalid port address on the remote host. For the first sequence of three
datagrams, a TTL field value is set to 1. The TTL value of 1 causes the
datagram to time out at the first router in the path. This router then responds
with an ICMP Time Exceeded Message (TEM) indicating that the datagram has
expired.
Three more UDP
messages are now sent, this time with the TTL value set to 2. This causes the
second router to return ICMP TEMs. This process continues until the packets
actually reach the other destination or the maximum TTL has been reached. The
default maximum TTL for traceroute is 30.
Since these
datagrams are trying to access an invalid port at the destination host, ICMP
Port Unreachable Messages are returned instead of ICMP TEMs. This indicates an
unreachable port and signals the Traceroute program to end the process.
The Lab Activity
will help students become more familiar with the traceroute command.
The next page
will discuss some other commands that are used to troubleshoot routing issues.
20.3 Troubleshooting Router Issues Overview
20.3.5 Troubleshooting routing issues
This page will
describe the show ip protocols and show ip route commands. These commands
display information about routing protocols and the routing table. The output
from these commands can be used to verify the routing protocol configuration.
The show ip route
command may be the most important command used to troubleshoot routing issues.
This command displays the contents of the IP routing table. The output from the
show ip route command shows the entries for all known networks and subnetworks,
and how that information was learned.
If there is a problem reaching
a host in a particular network, then the output of the show ip route command
can be used to verify that the router has a route to that network.
If the output of
the show ip route command does not show the expected learned routes, or any
learned routes, then the problem may be that routing information has not been
exchanged. In this case, use the show ip protocols command on the router to
check for a routing protocol configuration error.
The show ip
protocols command displays values about IP routing protocol information on the
entire router. This command can be used to confirm which protocols are
configured, which networks are being advertised, which interfaces are sending
updates, and the sources of routing updates. The show ip protocols output also
shows timers, filters, route summarization, route redistribution, and other
parameters that are specific to each routing protocol that is enabled on the
router. When multiple routing protocols are configured, the information about
each protocol is listed in a separate section.
The show ip
protocols command output can be used to diagnose many routing issues. For
example, it can be used to identify a router that may advertise incorrect
routing information. This command may be used to confirm that expected
protocols, advertised networks, and routing neighbors are present. It is
important to have documentation that indicates the expected results, or
baseline information, when a problem occurs. It may be impossible to identify
the problem without such documentation.
The Lab Activity
will teach students how to use the show ip route and show ip protocols command.
The next page
will discuss the show controllers serial command.
20.3 Troubleshooting Router Issues Overview
20.3.6 Troubleshooting using show controllers
This page will
teach students about troubleshooting using the show controllers command.
Router
configuration and troubleshooting can be performed remotely when physical
inspection of the connections is not possible. The show controllers command is
useful to determine the type of cable connected without inspecting the cables.
The output
displayed by the show controllers command indicates the type of cable detected
by the controller. This is useful for finding a serial interface with no cable,
the wrong type of cable, or a defective cable.
The show controllers serial 0/0
command queries the integrated circuit, or controller chip, that controls the
serial interfaces and displays information about the physical interface serial
0/0. This output varies for different controller chips.
Regardless of the
controller chip type, the show controllers command produces a large amount of
output. Other than the cable type, most of this output is internal technical
detail about the controller chip status. Without specific knowledge of the
integrated circuit, this information is of little use.
The Lab Activity
will show students how to troubleshoot with the show controllers command.
The Interactive
Media Activity will help students link network problems to the corresponding
layer of the OSI model.
The next page
will introduce the debug command.
20.3
Troubleshooting Router Issues Overview
20.3.7 Introduction to debug
This page will
explain the functions of the debug command.
The debug commands assist in
the isolation of protocol and configuration problems. The debug command is used
to display dynamic data and events. Since the show commands only display static
information, they provide a historical picture of the router operation. The
debug command output gives more insight into the current events of the router.
These events could be traffic on an interface, error messages generated by
nodes on the network, protocol-specific diagnostic packets, and other useful troubleshooting
data. The dynamic output of the debug
command creates performance issues. This command produces high processor
overhead that may disrupt normal router operation. For this reason, debug
should be used conservatively. Use debug commands to examine specific types of
traffic or problems after likely problems have been narrowed a few causes. The
debug command should be used to isolate problems and not to monitor normal
network operation.
WARNING:
The debug all
command should be used sparingly as this can disrupt router operations.
By default, the
router sends the debug output and system messages to the console. If a Telnet
session is used to examine the router, then the debug output and system
messages can be redirected to the remote terminal. This is done through the
Telnet session with the terminal monitor command. Use extra caution when the
debug commands are selected from a Telnet session. No command should be
selected that will cause the debug output to create additional traffic that creates
debug output. If this occurs, the Telnet session will rapidly saturate the link
with traffic or the router will exhaust one or more resources. A good rule to
follow to prevent this recursion of traffic is to never debug any activity on
the port where the session is established.
The output of the
different debug commands varies. Some may frequently generate many lines while
others produce a line or two of output every few minutes.
Another IOS
software service that will enhance the usefulness of the debug output is the
timestamps command. This command will put a timestamp on a debug message. This
information provides the time when the debug event occurred and the duration of
time between events.
This is often
very useful when troubleshooting intermittent problems. By time stamping the
output, a pattern of occurrence is often recognized. This helps to isolate the
source of the problem. This also prevents the technician from intently watching
the debug output for what may seem like hours.
The following
command configures a timestamp that will show the hour:minute:second of the
output, the amount of time since the router was last powered up, or when a
reload command was executed:
GAD(config)#service
timestamps debug uptime
The output from
this is useful to determine the time between events. To determine how long
since the last occurrence of the debug event, the time since the last reload
has to be used as a reference. This time can be found with the show version
command.
A more practical
use of the timestamps is to have it display the time and date that the event
occurred. This will simplify the process of determining the last occurrence of
the debug event. This is done using the datetime option:
GAD(config)#service
timestamps debug datetime localtime
It should be
noted that this command is only useful if the clock is set on the router.
Otherwise, the timestamp shown in the debug output is not an accurate time. To
ensure that the timestamps are correct, the router clock should be set to the
correct time from privileged EXEC mode with the following command:
GAD#clock set
15:46:00 3 May 2004
NOTE:
On some Cisco
platforms, the router clock is not backed up with a battery source, so the
system time will need to be reset after a router reload or power failure.
The no debug all
and undebug all commands turn off all diagnostic output. To disable a
particular debug command, use the no form of the command. For example, if the
debug ip rip command is used to monitor RIP, it can be disabled with no debug
ip rip. To view what is currently being examined by a debug command, use show
debugging.
The Lab Activity
will help students become more familiar with the debug command.
This page
concludes this lesson. The next page will summarize the main points from this
module.
Summary
This page summarizes the topics
discussed in this module.
The show ip route
command is used to gather detailed information about the routes installed on
the router. It displays the contents of the IP routing table. New routes may be
added with static routing, which allows an administrator to manually define
routes, or with dynamic routing, which uses the rules defined by a routing
protocol to exchange information and determine the best path.
Default routes
are used when the router is unable to match a destination network with a more
specific entry in the routing table. The router uses this default route to
reach the gateway of last resort in an effort to forward the packet.
The path
determination function occurs at the network layer of the OSI model. It enables
a router to evaluate the available paths to a destination and to establish the
preferred handling of a packet. This information is configured manually or
collected dynamically. The administrative distance of the route is used by the
router to decide what the best path is to a particular destination. The
administrative distance is a number that measures the trustworthiness of the
source of the route information. Lower administrative distances indicate more
trustworthy sources.
To determine the
best route to a destination, routing protocols use a value that measures the
desirability of a route called a metric. The metric is usually determined by
factors such as hop count, bandwidth, delay, load, reliability, and cost. Typically,
the smaller the metric number, the better the path.
Troubleshooting
should be an orderly process based on the networking standards set in place by
an administrator. Documentation is an important part of the troubleshooting
process. Indicator lights are a useful tool for troubleshooting at Layer 1. At
Layer 3, ping is used to test network connectivity. Telnet connections are used
to verify the application layer software between a source and a destination.
The show
interfaces command shows the status of two important portions of the
interfaces. They are the physical or hardware portion and logical or software
portion. These can be related to the Layer 1 and the Layer 2 functions. If the
interface is up and the line protocol is down, a Layer 2 problem exists. If the
physical layer is properly functioning, then all other directly connected Cisco
devices should be displayed. If no known device appears, a Layer 1 problem
likely exists.
To debug
connectivity issues, the show cdp neighbors command is used to display
information about directly connected neighbors. Use the traceroute command to
trace the routes that packets take to destinations. This command can be used to
test the network layer at each hop and provide performance benchmarks.
To verify the routing
protocol configuration, use the show ip protocols and show ip route commands.
These commands display information about routing protocols and the routing
table. To determine the type of cable connected without inspecting the cables,
use the show controllers command.
The debug command
is used to display dynamic data and events. Since the show commands only
display static information, they provide a historical picture of the router
operation. The debug command output gives more insight to the current events of
the router.
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