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Figure 2 Tracert 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 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 the
source device the address (1.1.2.2) of the second Layer 3 device.
5. The process continues until the ultimate destination device is reached. No application of the
destination uses this UDP port. 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 it can get the addresses of all Layer 3 devices involved to
get to the destination device (1.1.1.2, 1.1.2.2, 1.1.3.2).
Configuring tracert
Configuration prerequisites
Before you configure tracert, complete the following tasks:
• Enable sending of ICMP timeout packets on the intermediate device (the device between the source
and destination devices). If the intermediate device is an HP device, execute the ip ttl-expires enable
command on the device. For more information about this command, see IP performance
optimization commands in the Layer 3 - IP Services Command Reference.
• Enable sending of ICMP destination unreachable packets on the destination device. If the
destination device is an HP device, execute the ip unreachables enable command. For more
information about this command, see IP performance optimization commands in the Layer 3 - IP
Services Command Reference.
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