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. Since these datagrams are trying
to access an invalid port at the destination host, ICMP Port
Unreachable Messages are returned instead of the ICMP Time
Exceeded Message. This indicates an unreachable port and
signals the traceroute program, terminating the process.
Lab Activity Lab Exercise: Troubleshooting using
Traceroute This lab is to use the traceroute or
tracert command to verify that the network-layer between
source, destination and each router along the way is working
properly. Web Links Using the Extended ping and
Extended traceroute Commands
http://www.cisco.com/en/US/t ech/tk826/tk365/
technologies_tech_ note09186a0080093f22.shtml
Content
9.3 Troubleshooting Router Issues
Overview 9.3.5 Troubleshooting routing
issues The show ip protocols and show ip
route 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 is perhaps the single most important
command for troubleshooting 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 no learned routes,
then the problem is possibly that routing information is not
being 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 a multitude of routing issues,
including identifying a router that is suspected of delivering
bad router information. It may be used to confirm the expected
protocols, advertised networks, and routing neighbors are
present. As with any troubleshooting process, identifying the
problem is difficult if not impossible if there is not
documentation indicating the expected. Lab Activity Lab
Exercise: Troubleshooting Routing Issues with show ip
route and show ip protocols This lab uses the
show ip route and show ip protocols commands to
diagnose a routing configuration problem. Web Links
show ip protocols Command
http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/
products_command_reference_ chapter09186a00800ca759.html#1039245
Content 9.3 Troubleshooting
Router Issues Overview 9.3.6 Troubleshooting
using show controllers serial Very often, configuration and
troubleshooting on routers is done remotely when it is not
possible to physically inspect the router connections. The
show controllers serial command is useful to determine
the type of cable connected without inspecting the cables. By
examining the show controllers serial command output,
the type of cable that the controller detects can be
determined. This is useful for finding a serial interface with
no cable, the wrong type of cable, or a defective cable. The
show controllers serial command queries the integrated
circuit (chip) that controls the serial interfaces and displays
information about the physical interface. This output varies
from controller chip to controller chip. Even within a router
type, different controller chips may be used. Regardless of the
controller type, the show controllers serial command
produces a tremendous amount of output. Other than the cable
type, most of this output is internal technical detail
regarding the controller chip status. Without specific
knowledge of the integrated circuit, this information is of
little use. Lab Activity e-Lab Activity:
Troubleshooting Using show controllers serial In this
lab, the students will use the show controllers command
to see what cable is attached to the serial interface. Web
Links show controllers serial Command
http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/
products_command_reference_ chapter09186a00800874c2.html#1019005
Content 9.3 Troubleshooting
Router Issues Overview 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. 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 comes at a performance cost, producing 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.
debug should be used 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 being 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 by
issuing the terminal monitor command. Extra caution
should be used when selecting the debug commands from a