Module 9: Basic Router
Troubleshooting
Frank Mann CCAI-CCNA
2
9.1 Examining the Routing
Table
Frank Mann CCAI-CCNA
3
9.1.1 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, also called routes, from an
administrator's configuration or from other
routers by way of routing protocols. Routers
store this routing information in routing tables
using on-board dynamic random access memory
(DRAM).
A routing table contains a list of the best available
routes. Routers use the routing table to make
packet forwarding decisions.
Displaying the
IP Routing Table
9.1.2 Determining the gateway of last
resort
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
Default Routes
• This route allows the stub network to reach all known
networks beyond router A.
Default Routes
• This route allows the stub network to reach all known
networks beyond router A.
Ip Route
AD = ?
Static Routes
• The following are advantages of using a static routes
•Secure operation
•Low processor overhead
•Precise control of path selection
RIP Overview
• Maximum is 6 paths (default = 4)
• Hop-count metric selects the path
• Routes update every 30 seconds
9.1.3 Determining route source and
destination
The Switching Process
Consider the process of moving a packet that is received on
the Ethernet interface and destined for the FDDI interface.
This example describes the switching process. First, the
frame arrives at the Ethernet interface as a Layer 2 MAC
frame.
MAC Header
The router immediately removes the MAC header
and looks into the packet header for the destination
network.
Lookup
A lookup into the particular protocol's routing table
indicates that the packet with that destination
network address is destined for the FDDI interface.
New FDDI MAC
The new FDDI MAC frame must now be built for
transmitting out of that interface, and the router
must determine the necessary destination MAC
address to insert into the new FDDI frame.
9.1.4 Determining L2 and L3
addresses
9.1.5 Determining the route
administrative distance
The administrative distance
of the route is the key
information that the router
uses in deciding which is
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
9.1.6 Determining the route metric
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.
Typically, the smaller the metric number, the better
the path.
9.1.6 Determining the route metric
IGRP calculates the metric by adding the weighted values of different characteristics of
the link to the network in question. In the following example the values bandwidth,
bandwidth divided by load, and delay, are weighted with the constants K1, K2, and K3.
Metric=K1 *Bandwidth + (K2 * Bandwidth)/256-load) + K3 * Delay
The default constant values are K1=K3=1 and K2=K4=K5=0 so:
Metric=Bandwidth + Delay
9.1.7 Determining the route next hop
9.1.8 Determining the last routing
update –show ip protocols
Update Timers
Networks being
Advertised
Sources of routing
updates
9.1.9 Observing multiple paths to
destination
Some routing protocols support multiple
paths to the same destination.
Unlike single path algorithms, these multipath algorithms permit traffic over
multiple lines, provide better throughput,
and are more reliable
9.2 Network Testing
Frank Mann CCAI-CCNA
25
9.2.1 Introduction to network testing
Basic testing of a network should proceed in
sequence from one OSI reference model layer to
the next.
It is best to begin with Layer 1 and work to Layer 7
if necessary.
Beginning with Layer 1, look for simple problems
such as power cords plugged in the wall. 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.
9.2.2 Using a structured approach to
troubleshooting
Using a structured approach
to troubleshooting, every
member of a network
support team can know what
steps each member of the
team has completed to solve
a problem.
if a variety of
troubleshooting ideas are
tried with no organization or
documentation, problem
solving is not efficient.
9.2.3 Testing by OSI layers-physical
Layer 1 errors can include:
• Broken cables
• Disconnected cables
• Cables connected to the
wrong ports
• Intermittent cable
connection
• Wrong cables used for the
task at hand (must use
rollovers, crossover cables,
and straight-through cables
correctly)
• Transceiver problems
• DCE cable problems
• DTE cable problems
• Devices turned off
9.2.3 Testing by OSI layers-Data Link
Layer 2 errors can
include:
• Improperly configured
serial interfaces
• Improperly configured
Ethernet interfaces
• Improper encapsulation
set (HDLC is default for
serial interfaces)
• Improper clockrate
settings on serial
interfaces
• Network interface card
(NIC) problems
9.2.3 Testing by OSI layers-Network
Layer 3 errors can
include:
• Routing protocol not
enabled
• Wrong routing protocol
enabled
• Incorrect IP addresses
• Incorrect subnet masks
9.2.4 Layer 1 troubleshooting using
indicators
9.2.5 Layer 3 troubleshooting using
ping
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.
9.2.6 Layer 7 troubleshooting using
Telnet
•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.
9.2.6 Layer 7 troubleshooting using
Telnet
•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.
9.3 Troubleshooting Router
Issues Overview
Frank Mann CCAI-CCNA
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9.3.1 Troubleshooting Layer 1 using
show interface
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
Layer 1 indicators
There could be a
problem with faulty
telephone equipment.
• A faulty switch or
CSU/DSU
The router hardware
could be defective.
There could be a
problem with the
cabling.
There could be a
problem from the ISP
Layer 1 indicators-Counters
9.3.2 Troubleshooting Layer 2 using
show interface
The show interfaces command is perhaps the single most
important tool to discover Layer 1 and Layer 2 problems
with the router.
• The first parameter (line) refers to the physical layer.
• The second parameter (protocol) indicates whether the IOS
processes that control the line protocol consider the interface
usable. This is determined by whether keepalives are
successfully received.
9.3.3 Troubleshooting using show cdp
Cisco Discovery Protocol (CDP) advertises device
information to its direct neighbors, including MAC and IP
addresses and outgoing interfaces. The output from the
show cdp neighbors command displays information about
directly connected neighbors.
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.
9.3.4 Troubleshooting using
traceroute
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
9.3.5 Troubleshooting routing issues
9.3.5 Troubleshooting routing issues
9.3.6 Troubleshooting using show
controllers serial
9.3.7 Introduction to debug
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.
Using the debug command output gives
more insight to the current events of the
router.
show and debug Commands
show
debug
Processing characteristic
Static
Dynamic
Processing load
Low overhead
High overhead
Primary use
Gather facts
Observe processes
Considerations When Using debug
Commands
• May generate output in a variety of formats that
may not identify the problem
• Require high overhead, possibly disrupting
network device operation
• Useful for obtaining information about network
traffic and router status
The Importance of show and debug
Commands
The show and debug commands are important tools for
troubleshooting that are built into Cisco IOS software. The
show commands snapshot problems with interfaces, media,
or network performance, whereas debug commands check
the flow of protocol traffic for problems, protocol bugs, or
misconfigurations.
Examining show and debug
Commands
This section describes guidelines for using show
and debug commands.
The Importance of show and debug
Commands
The show and debug commands are important tools for
troubleshooting that are built into Cisco IOS software. The
show commands snapshot problems with interfaces, media,
or network performance, whereas debug commands check
the flow of protocol traffic for problems, protocol bugs, or
misconfigurations.
The show Command
The show command provides a static collection of
information about the status of a router, neighboring routers,
and network performance. Use show commands when you
are gathering facts for isolating problems in an internetwork,
including problems with router interfaces, nodes, media,
servers, clients, or applications.
The show Command
The show command provides a static collection of information about the
status of a router, neighboring routers, and network performance. Use
show commands when you are gathering facts for isolating problems in an
internetwork, including problems with router interfaces, nodes, media,
servers, clients, or applications.
The debug Command
The debug command provides a flow of information about the
traffic being seen (or not seen) on an interface, error messages
generated by nodes on the network, protocol-specific diagnostic
packets, and other useful troubleshooting data. Use debug when
you want to see a process operating on the router or network to
determine if events or packets are working properly.
Guidelines for Optimum
Effectiveness
Use debug commands to isolate problems, not to monitor
normal network operation. Because the high overhead of
debug commands can disrupt router operation, you should
use debug commands only when you are looking for specific
types of traffic or problems and have narrowed your
problems to a likely subset of causes.
9.3.7 Introduction to debug
9.3.7 Introduction to debug
Considerations for Disabling Debugging
Suggestions for disabling debugging are shown
here.
When to Terminate debug
In all cases, when you finish using a debug command,
remember to disable it with its specific no debug command
or with the no debug all command.
No Visible Prompt
Sometimes debug can output a considerable volume of data
to a terminal. Even if there is no prompt visible on your
terminal, you can still type in the command to turn off the
debug. In all cases, turn off all debugging with undebug all or
no debug all as soon as you are finished troubleshooting.
Network Analyzer
No single tool works best in all cases. In some cases,
attaching a network analyzer to the suspect network is less
intrusive and more likely to yield applicable information
without exacerbating load problems for a router.
Labs Module 9: Basic Router Troubleshooting
Lab:
9.1.1 Using Show IP Route to Examine Routing Tables
9.1.2 Gateway of Last Resort
9.1.8 Last Route Update
9.2.6 Troubleshooting Using Ping and Telnet
9.3.4 Troubleshooting using Traceroute
9.3.5 Troubleshooting Routing Issues with show ip route and show ip protocols
9.3.7 Troubleshooting Routing Issues with Debug
e-Lab:
9.1.5 Administrative Distance
9.2.5 Layer 3 Troubleshooting Using ping
9.3.1 Troubleshooting Layer 1: Using show interface
9.3.3 Cisco Discovery Protocol
9.3.6 Troubleshooting Using show controllers serial