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 command to
reset the counters to zero. These counters should always be
cleared after an interface problem has been corrected. Starting
from zero gives a better picture of the current status of the
network and will help verify that the issue has indeed been
corrected. Lab Activity e-Lab Activity: Troubleshooting
Layer 1: Using show interfaces In this lab, the students
will verify that the physical serial 0/0 link is up, reset the
interface counters, and check the interface again to see what
happens after a counter reset. Web Links show ip
interfaces Command http://www.cisco.com/en/US/products/sw/
iosswrel/ps1835/ products_command_reference_
chapter09186a0080087387.html#1020817
Content
9.3 Troubleshooting Router Issues
Overview 9.3.2 Troubleshooting Layer 2 using
show interfaces 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. 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. Among the possible
causes are: - No keepalives
- No clock rate
- Mismatch in encapsulation type
The show
interfaces serial command should be used after configuring
a serial interface to verify the changes and that the interface
is operational. Interactive Media Activity Drag and
Drop: show interface interface command After
completing this activity, the student will be able to
understand show interface interface command.
Web Links show ip interface EXEC Command
http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/
products_command_reference_ chapter09186a0080087387.html#1020817
Content 9.3 Troubleshooting
Router Issues Overview 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. 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 is
properly functioning, then all other directly connected Cisco
devices should be displayed. If no known device appears, a
Layer 1 problem likely exists. 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. Lab Activity e-Lab Activity: Cisco
Discovery Protocol In this lab, the students will use CDP to
obtain information about neighboring Cisco devices. Web
Links show cdp privileged EXEC Command
http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/
products_command_reference_ chapter09186a00800ca747.html#1019270
Content 9.3 Troubleshooting Router
Issues Overview 9.3.4 Troubleshooting using
traceroute The traceroute command is used to
discover the routes that packets take when traveling to their
destination. Traceroute can also be used to help test the
network layer (Layer 3) on a hop-by-hop basis 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
is occurring. For each router in the path a line of output is
generated on the terminal indicating the IP address of the
interface that the data entered. If an asterisk (*) appears,
the packet failed. By obtaining the last good hop from the
traceroute output and comparing it to a diagram of the
internetwork, the problem area can be isolated. Traceroute also
provides information indicating the relative performance of
links. The round trip time (RTT) is the time required to send
an echo 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. In order for the traceroute or
ping data to successfully make the round trips between
routers, there have to 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 sent, a
Time-To-Live (TTL) field value is set to one. The TTL value of