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| 3.3 |  | Windows and Cisco Commands for Physical Layer Information Gathering |
| 3.3.1 |  | End-system commands – common commands |

In order to gather information on a physical layer problem it is important to be familiar with end-station commands that exist for this purpose. Interrogating devices at the periphery of the network is a good idea when end-systems are experiencing connectivity problems.

There are a number of commands that are common to the popular operating platforms like Windows, UNIX and Mac OS. These can be used to ascertain whether an end station is achieving connectivity with the network.

The ping {host | 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.