Troubleshooting using show controllers 9.3.6 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 ca...
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Troubleshooting routing issues 9.3.5 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 ...
Troubleshooting using show cdp 9.3.3 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 a...
Troubleshooting Layer 2 using show interfaces 9.3.2 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 ...
Troubleshooting Layer 1 using show interfaces 9.3.1 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 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 ...
Layer 7 troubleshooting using Telnet 9.2.7 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 ...
Layer 1 troubleshooting using indicators 9.2.4 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 con...