H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 13)

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Figure 2 Network diagram
Tracert uses received ICMP error messages to get the IP addresses of devices. As shown in Figure 2 ,
tracert works as follows:
1. The source device (Device A) sends a packet with a TTL value of 1 to the destination device (Device
D). The UDP port of the packet is a port number not used by any application on the destination
device.
2. The first hop (Device B, the first Layer 3 device that 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 can get the address of the first Layer 3 device (1.1.1.2) .
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 of the second Layer 3 device (1.1.2.2) .
5. This process continues until a packet sent by the source device reaches the ultimate destination
device. Because no application uses the destination port specified in the packet, the destination
device responds with a port-unreachable ICMP message to the source device, with its IP address
encapsulated. This way, the source device gets the IP address of the destination device (1.1.3.2).
6. The source device thinks that the packet has reached the destination device after receiving the
port-unreachable ICMP message, and the path to the destination device is 1.1.1.2 to 1.1.2.2 to
1.1.3.2.
Prerequisites
Before you use a tracert command, perform the tasks in this section.
• Enable sending of ICMP timeout packets on the intermediate devices (devices between the source
and destination devices). If the intermediate devices are H3C devices, execute the ip ttl-expires
enable command on the devices. For more information about this command, see Layer 3 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 Command Reference.