4
Figure 2 Network diagram
The tracert function is implemented through ICMP, as shown in Figure 2:
1. The source (Device A) sends a packet with a TTL value of 1 to the destination (Device D). The UDP
port of the packet is a port number that will not be used by any application of the destination.
2. The first hop (Device B) (the Layer 3 device that first receives the packet) responds by sending a
TTL-expired ICMP error message to the source, with its IP address 1.1.1.2 encapsulated. In this
way, the source device receives 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 the
source 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 of the
destination uses this UDP port. Therefore, the destination replies a port unreachable ICMP error
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 the
packet has reached the destination, and that it has received the addresses of all the Layer 3
devices involved in delivering the packet to the destination device (1.1.1.2, 1.1.2.2, 1.1.3.2).
Configuring tracert
Configuration prerequisites
For an IPv4 network:
• Enable sending of ICMP timeout packets on the intermediate device (the device between the source
and destination devices). If the intermediate device is an H3C device, execute the ip ttl-expires
enable command on the device. For more information about this command, see Layer 3—IP
Services Command Reference.
• Enable sending of ICMP destination unreachable packets on the destination device. If the
destination device is an H3C device, execute the ip unreachables enable command. For more
information about this command, see Layer 3—IP Services Command Reference.
For an IPv6 network:
• Enable sending of ICMPv6 timeout packets on the intermediate device (the device between the
source and destination devices). If the intermediate device is an H3C device, execute the ipv6
hoplimit-expires enable command on the device. For more information about this command, see
Layer 3—IP Services Command Reference.
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