The S2700 implements tracert based on ICMP. Tracert records the gateways that the ICMP
message passes along the path between a source host and a destination. In this manner, you can
check network connectivity and locate the fault.
Figure 5-3
Principle of the tracert operation
Switch
Router-A
Router-B
Log Host
Step 1
TTL=1
TTL=2
TTL=3
UDP datagram
ICMP Time Exceeded message
ICMP Destination Unreachable message
Step 2
Step 3
as an example to show tracert implementation on the
S2700. On the S2700, run the
tracert
command. The destination IP address is the IP address of
the log host and other parameters adopt the default values.
1.
The S2700 sends a UDP datagram to the log host, with the TTL value being 1 and the
destination UDP port number being 33434.
2.
After receiving the UDP datagram from the S2700, Router-A finds that the destination IP
address carried in the datagram is not its own address. Then, Router-A reduces the TTL
value by 1. Finding that the TTL value reaches 0, Router-A sends an ICMP Time Exceeded
message to the S2700.
3.
After receiving the ICMP Time Exceeded message, the S2700 increases the TTL value and
the UDP port number in the UDP datagram by 1 respectively and then sends out the UDP
datagram again.
4.
Perform Step 2 and Step 3, the log host receives the UDP datagram from the S2700.
5.
After receiving the UDP datagram from the S2700, the log host finds that the destination
is itself. It begins to process the datagram. The log host tries to find the upper layer protocol
corresponding to the destination UDP port number carried in the datagram. In most cases,
the UDP ports whose number is greater than 30000 are not used by any protocols. Therefore,
the log host sends an ICMP Destination Unreachable message to the S2700 to notify the
source that the destination port is unreachable.
6.
After receiving the ICMP Destination Unreachable message from the log host, the S2700
knows that the UDP datagram has reached the destination and thus stops running the tracert
program.
In the preceding steps, the tracert program on the source records the IP addresses of the gateways
between the source and the destination through the ICMP Time Exceeded message mentioned
in Step 3.
5.3 Performing Ping and Tracert Operations
This section describes the execution of the
ping
and
tracert
commands.
Quidway S2700 Series Ethernet Switches
Configuration Guide - Network Management
5 Ping and Tracert
Issue 01 (2011-07-15)
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
247
