H3c-technologies H3C S5120 Series Switches User Manual download pdf (Page 623)

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5) Upon receiving the reply, the source device adds the IP address (1.1.1.1) of its inbound interface to

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.

Tracert

Introduction

By using the tracert command, you can trace the Layer 3 devices involved in delivering an IP packet

from source to destination to check whether a network is available. This is useful for identification of

failed node(s) in the event of network failure.

Figure 1-2 Tracert diagram

The tracert function is implemented through ICMP, as shown in Figure 1-2

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 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 it can get the addresses of all the Layer 3 devices involved

to get to the destination device (1.1.1.2, 1.1.2.2, 1.1.3.2).

Configuring Tracert

Follow these steps to configure tracert:

To do… Use the command… Remarks

Enter system view

system-view

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