1-11 Diagnostic ToolsOverviewPingYou can use the ping function to check whether a device with a specified address is reachable, and toexamine network connectivity.A successful execution of the ping command involves the following steps:1) The source device sends an ICMP echo request (ECHO-REQUEST) to the destination device.2) The destination device responds by sending an ICMP echo reply (ECHO-REPLY) to the sourcedevice after receiving the ICMP echo request.3) The source device displays related statistics after receiving the reply.Output of the ping command falls into the following:z If the source device does not receive an ICMP echo reply within the timeout time, it displays theprompt information and the statistics during the ping operation.z If the source device receives an ICMP echo reply within the timeout time, it displays the number ofbytes of the echo reply, the message sequence number, Time to Live (TTL), the response time,and the statistics during the ping operation. Statistics during the ping operation include number ofpackets sent, number of echo reply messages received, percentage of messages not received,and the minimum, average, and maximum response time.Trace RouteBy using the trace route command, you can display the Layer 3 devices involved in delivering a packetfrom source to destination. This function is useful for identification of failed node(s) in the event ofnetwork failure.The trace route command involves the following steps in its execution:1) The source device sends a packet with a TTL value of 1 to the destination device.2) The first hop (the Layer 3 device that first receives the packet) responds by sending a TTL-expiredICMP message to the source, with its IP address encapsulated. In this way, the source device canget the address of the first Layer 3 device.3) The source device sends a packet with a TTL value of 2 to the destination device.4) The second hop responds with a TTL-expired ICMP message, which gives the source device theaddress of the second Layer 3 device.5) The above process continues until the ultimate destination device is reached. In this way, thesource device can trace the addresses of all the Layer 3 devices involved to get to the destinationdevice.
