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: 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