21-45) Upon receiving the reply, the source device adds the IP address (1.1.1.1) of its inbound interfaceto the RR option. Finally, you can get the detailed information of routes from Device A to Device C:1.1.1.1 <-> {1.1.1.2; 1.1.2.1} <-> 1.1.2.2.TracertIntroductionBy using the tracert command, you can trace the Layer 3 devices involved in delivering an IP packetfrom source to destination to check whether a network is available. This is useful for identification offailed node(s) in the event of network failure.Figure 21-2 Tracert diagramThe tracert function is implemented through ICMP, as shown in Figure 21-2:2) 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 the destination.3) The first hop (Device B) (the Layer 3 device that first receives the packet) responds by sending aTTL-expired ICMP error message to the source, with its IP address 1.1.1.2 encapsulated. In thisway, the source device can get the address (1.1.1.2) of the first Layer 3 device.4) The source device sends a packet with a TTL value of 2 to the destination device.5) 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.6) The above process continues until the ultimate destination device is reached. No application of thedestination uses this UDP port. Therefore, the destination replies a port unreachable ICMP errormessage with the destination IP address 1.1.3.2.7) 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 TracertFollow these steps to configure tracert:To do… Use the command… RemarksEnter system view system-view —
