HP VPN Firewall Appliances System Management and Maintenance Configuration Guide

Figure 6 Traceroute operation

Tracert uses received ICMP error messages to get the IP addresses of devices. As shown in Figure 6,

tracert works as follows:

1. The source device (Device A) sends a UDP packet with a TTL value of 1 to the destination device

(Device D). The destination UDP port is 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. The process continues until the 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.

Configuring tracert in the Web interface

The Web interface does not support IPv6 traceroute.

Before performing the traceroute operation, execute the ip ttl-expires enable command on intermediate

devices to enable the sending of ICMP timeout packets and execute the ip unreachables enable

command on the destination device to enable the sending of ICMP destination unreachable packets.

To perform a traceroute operation:

1. Select Network > Diagnostic Tools from the navigation tree.

2. Select the Trace Route tab to enter the traceroute configuration page, as shown in Figure 7.