ip-address} command is used to verify connectivity between hosts by sending an ICMP echo request to the target IP address. If all host unreachable results are returned to the sending station, this indicates that the station does not have connectivity to the network. It is typical to ping the directly attached router interface of a LAN segment, then ping progressively further into the network to establish the point at which loss of connectivity occurs. The Arp –a is a very useful interrogatory command because it establishes whether there is Layer 2 and Layer 3 connectivity within a LAN segment. The ARP table is flushed and renewed periodically. If this command does not return a list of MAC addresses and associated IP addresses then the loss of connectivity is very likely to be at the physical layer. This would also be the case if the MAC address corresponding to the attached router interface was not present. While peer machines are still present in an ARP table the link to the router interface or the interface itself is likely to be the problem.
Content 3.3 Windows and Cisco Commands for Physical Layer Information Gathering 3.3.2 End-system commands – Windows only For Windows end systems, the user often queries a network connection at the MS-DOS command prompt. By entering commands at the DOS command prompt, or by using the Run window in the Start menu, a user can establish whether a connection is present with other network devices. Also the path that data takes to reach the destination can be determined, which can be used to verify multiple links and also the reliability of any connection. The ipconfig /all command is a simple way to check connectivity on a Windows NT/2000/XP system. It will identify the host MAC address, DNS, DHCP, NT and WINS servers that the host is attached to if it has physical connectivity. The tracert command can be used to test connectivity to a destination device. It will enable a user to map the route taken on the way to the destination. The winipcfg command is used in older version of 9x Windows up to Me. As the name suggests winipcfg will show the Windows IP configuration information.
Content 3.3 Windows and Cisco Commands for Physical Layer Information Gathering 3.3.3 End-system commands – UNIX/Mac OS For UNIX or Mac OS end-systems the user often queries the status of the network connection by using run-mode to execute network connectivity queries. By entering commands a user can establish whether a connection is present with other network devices, the path that data takes to reach the target and also the reliability of any connection. The ifconfig –a command performs the same function that the ipconfig command performs for Windows NT/2000/XP systems. It will list the IP address information for a Mac OS X and UNIX hosts. The traceroute command can be used to show the path a packet takes through the network. It is useful to identify at what point a link is broken, or sub-optimal, in the network.
Content 3.3 Windows and Cisco Commands for Physical Layer Information Gathering 3.3.4 Common Cisco IOS commands A variety of commands exist under the Cisco IOS to gather information on connectivity problems. When a router experiences an absence or intermittent loss of connectivity to other network components, the device can be interrogated for statistics collected on a port or interface. Additionally networking processes can be monitored in real-time to determine at what point in the sequence connectivity is lost. In the case where numerous routes exist to a desired destination, which is suffering intermittent or total loss of connectivity, information on the actual point at which the break occurs can be collected.
Content 3.3 Windows and Cisco Commands for Physical Layer Information Gathering 3.3.5 Cisco IOS show commands Show commands represent the most complete set of tools in the IOS armory for collecting information on network outages. A very wide range of parameters can be examined individually, or in combination with other related parameters. Individual components in a router or switch can be examined, from a number of different angles. Use the show commands to gain insight to likely causes of loss of network connectivity. Not all of the commands listed are available on some versions of Cisco operating systems. To determine which commands are available for use with particular devices, consult the online documentation for Cisco devices at http://www.cisco.com/univercd/home/home.htm.
Content 3.4 Identifying Physical Layer Problems 3.4.1 Power related Power related issues are the most fundamental reason for network failure. The main AC power flows into either an external or internal AC to DC transformer module within a device. The transformer provides correctly modulated DC current, which acts to power device circuits, connectors, ports and the blowers used for device cooling. If a power related issue is suspected, a physical inspection of the power module is often carried out. Initially, with the power switch on, does the blower operate? Distribution and core layer Cisco devices are generally modular in nature. This approach can follow through for the power supply(s) of these units. Often parallel and backup power supplies exist in the chassis to balance the power load and to provide uninterrupted operation in the event of failure of a single or multiple online power modules. To help isolate a power subsystem problem, follow these steps: Step 1
Ensure that the power supply is flush with the back of the chassis. Step 2
Unplug the power cord, loosen and reinstall the power supply, tighten the captive installation screws, and then plug in the power cord. If this unit has more than one power cord, repeat the steps for each power supply.
Content 3.4 Identifying Physical Layer Problems 3.4.2 Cabling faults – CAT5 Many problems can be corrected by simply reseating cables that have become partially disconnected. When performing a physical inspection, look for damaged cables, improper cable types, and poorly crimped RJ-45s. Suspect drop cables should be subject to a simple cable test, and exchanged with a known-good cable. Do not assume that just because a cable is new, just out of the package, that it will work. Test it first. Cables made on site should also be tested before attempting to troubleshoot with them. Anyone can have a bad day and miswire the termination. Also test for simple cable faults such as shorts, opens, and split pairs. Determine whether a station connected by UTP or fiber is successfully linking to the network at the MAC Layer.