Cisco router trace command

At present, there is a high market demand for Cisco routers. So I studied the Cisco router trace command and shared it with you here. I hope it will be useful to you. The trace command provides information about each hop from the Cisco router to the destination address. It is implemented by controlling the TTL field of IP packets.

ICMP response request packets whose TTL is equal to 1 will be sent first. The first Cisco router in the path will discard the message and send a message that identifies the error message. An error message is usually an ICMP timeout message, indicating the next hop of the packet to reach the path smoothly or the port cannot reach the message, indicating that the message has been received by the destination address but cannot be transferred to the IP protocol stack. In privileged mode, extended Trace can be used to extend many attributes of the ping command. Special attributes of the extended trace command include:

Numeric display by default, the output of the trace command includes both the IP address and the corresponding DNS domain name. If you do not need to display the DNS domain name, you can use this attribute. The default value of Probe count is 3, which can be adjusted as needed. TTL value can be changed between the maximum and minimum TTL values. Port number is a very useful property that enables engineering and technical personnel to track specific transport layer ports. Therefore, you can not only check the IP connectivity between the source end and the target end, but also check whether the high-level service is accessible.

Another problem related to the trace command is that if multiple paths reach the destination, the source address of the returned message may be different. In this case, you need to carefully compare the delay time of different returned packets. If you still cannot obtain clear results, you can remotely access one or more Cisco routers in the path and use the trace command to access the Source and Destination addresses.

To obtain the round-trip delay time information, trace sends three packets and displays the average delay time. Add the TTL field of the packet to 1 and send three packets. These packets will arrive at the second Cisco router in the path and return a timeout error or port inaccessibility message. This method is used repeatedly to increase the value of the TTL field of the message until the Response Message of the target address is received.

In some cases, using the trace command may cause a fault. Because IOS has a bug related to the trace command. Information about these bugs can be obtained from CCO. Another problem is that some target sites do not respond to ICMP port inaccessible messages. When the command output displays a series of asterisks (*), such sites may be encountered. The user can interrupt command execution by using the Ctrl-Shift-6.

The output of a simple trace command in user execution mode is shown below. The distance to the destination is 3 hops. The Response Message of the three packets whose TTL value is 1 is an ICMP timeout error, and the IP addresses of the returned packets are two. Because CISCO vro1 1 and vro2 2 are in the same network segment and their distance to vro3 3 is a hop, these vrouters respond to the report.