TAILIEUCHUNG - Common Troubleshooting Tools

Common Troubleshooting Tools The most fundamental troubleshooting technique for network devices is to merely determine whether a device is reachable | Common Troubleshooting Tools The most fundamental troubleshooting technique for network devices is to merely determine whether a device is reachable. Simply put is the device up or is it down. To this end the Packet Internet Groper PING utility was developed to provide a simple way to determine the reachability of a device. Determining Reachability PING is built upon the Internet Control Message Protocol ICMP and uses a system of echo and echo-reply messages to indicate whether a host is reachable. When a source host attempts to determine whether a destination host is reachable it generates an ICMP echo packet for the destination host. When the destination host receives the echo packet it responds with an echo-reply packet allowing the source host to ascertain that the destination is reachable. If for some reason the destination host does not respond the assumption is that it is unreachable. It is important to note that this is an assumption because there are any number of reasons a host might not respond to an echo packet even though it is actually up and accessible. Therefore it is important to understand that the failure to receive a response to an echo message does not necessarily provide any information regarding what if anything might be wrong. It simply means that the source host did not receive a response to the echo message. In the event that a host fails to respond to an echo request two common messages may be generated. The first is a request timed out response. This typically indicates that the destination network was able to be reached but that the destination host did not respond which indicates a problem with the destination host itself . The other response is destination network unreachable. This message indicates that the destination network was unable to be located and typically indicates a problem with the network interconnectivity or a routing problem not necessarily a problem with the destination host. As mentioned in Chapter 3 TCP IP for .

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