1-4TracertIntroductionBy using the tracert command, you can trace the Layer 3 devices involved in delivering an IPpacket from source to destination to check whether a network is available. This is useful foridentification of failed node(s) in the event of network failure.Figure 1-2 Tracert diagramThe tracert function is implemented through ICMP, as shown in Figure 1-2:1) The source (Device A) sends a packet with a TTL value of 1 to the destination (Device D). TheUDP port of the packet is a port number that will not be used by any application of thedestination.2) The first hop (Device B) (the Layer 3 device that first receives the packet) responds by sendinga TTL-expired ICMP error message to the source, with its IP address 1.1.1.2 encapsulated. Inthis way, the source device can get the address (1.1.1.2) 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 (Device C) responds with a TTL-expired ICMP error message, which gives thesource device the address (1.1.2.2) of the second Layer 3 device.5) The above process continues until the ultimate destination device is reached. No application ofthe destination uses this UDP port. Therefore, the destination replies a port unreachable ICMPerror message with the destination IP address 1.1.3.2.6) When the source device receives the port unreachable ICMP error message, it knows that thepacket has reached the destination, and it can get the addresses of all the Layer 3 devicesinvolved to get to the destination device (1.1.1.2, 1.1.2.2, 1.1.3.2).Configuring TracertConfiguration prerequisitesz Enable sending of ICMP timeout packets on the intermediate device (the device between thesource and destination devices). If the intermediate device is an H3C device, execute the ipttl-expires enable command on the device. For more information about this command, see IPPerformance Optimization Configuration Commands in the Layer 3 - IP Services CommandReference.
