Troubleshooting using show cdp
9.3.3 This page will explain the functions of the show cdp command.
CDP advertises device information to its direct neighbors. This includes MAC and IP addresses and outgoing interfaces.
The output from the show cdp neighbors command displays information about directly connected Cisco device neighbors.
This information is useful for debugging connectivity issues. If a cabling problem is suspected, enable the interfaces with the no shutdown command and then execute the show cdp neighbors detail command before any other configuration. The command displays specific device detail such as the active interfaces, the port ID, and the device. The version of Cisco IOS that is running on the remote devices is also shown.
If the physical layer functions properly, then all other directly connected Cisco devices should be displayed. If no known device appears, this usually indicates a Layer 1 problem.
One area of concern with CDP is security. The amount of information CDP provides is so extensive that it can be a potential security hole. For security reasons CDP should be configured only on links between Cisco devices and disabled on user ports or links that are not locally managed.
In the Lab Activity, students will use the show cdp command to learn about network devices.
The next page will discuss the traceroute command.
9.3.3 This page will explain the functions of the show cdp command.
CDP advertises device information to its direct neighbors. This includes MAC and IP addresses and outgoing interfaces.
The output from the show cdp neighbors command displays information about directly connected Cisco device neighbors.
This information is useful for debugging connectivity issues. If a cabling problem is suspected, enable the interfaces with the no shutdown command and then execute the show cdp neighbors detail command before any other configuration. The command displays specific device detail such as the active interfaces, the port ID, and the device. The version of Cisco IOS that is running on the remote devices is also shown.If the physical layer functions properly, then all other directly connected Cisco devices should be displayed. If no known device appears, this usually indicates a Layer 1 problem.
One area of concern with CDP is security. The amount of information CDP provides is so extensive that it can be a potential security hole. For security reasons CDP should be configured only on links between Cisco devices and disabled on user ports or links that are not locally managed.
In the Lab Activity, students will use the show cdp command to learn about network devices.
The next page will discuss the traceroute command.
Troubleshooting using show traceroute
9.3.4 This page will describe the functions of the traceroute command.
The traceroute command is often referred to as the trace command in reference materials. However, the correct command syntax is traceroute. The traceroute command displays the routes that packets take to their destinations. The traceroute command can also be used to test each hop at the network layer and provide performance benchmarks.
The output of the traceroute command generates a list of hops that were successfully reached. If the data successfully reaches the intended destination, then the output indicates every router that the datagram passes through. This output can be captured and used for future troubleshooting of the internetwork.
Traceroute output will also indicate the specific hop at which the failure occurs. A line of output is generated on the terminal for each router in the path. This indicates the IP address of the interface that the data entered. If an asterisk (*) appears, the packet failed. To isolate the problem area, obtain the last good hop from the traceroute output and compare it to a diagram of the internetwork.
The traceroute command also provides information about the performance of links. The round trip time (RTT) is the time required to send an echo packet and get a response. This is useful for an approximate idea of the delay on the link. These figures are not precise enough to be used for an accurate performance evaluation. However, this output can be captured and used for future performance troubleshooting of the internetwork.
This is useful for an approximate idea of the delay on the link. These figures are not precise enough to be used for an accurate performance evaluation. However, this output can be captured and used for future performance troubleshooting of the internetwork.Notice that the device receiving the traceroute also has to know how to send the reply back to the source of the traceroute. For the traceroute or ping data to make the round trip between routers, there must be known routes in both directions. A failed response is not always an indication of a problem because ICMP messages could be rate-limited or filtered at the host site. This is especially true across the Internet.
Traceroute sends out a sequence of User Datagram Protocol (UDP) datagrams from the router to an invalid port address on the remote host. For the first sequence of three datagrams, a TTL field value is set to 1. The TTL value of 1 causes the datagram to time out at the first router in the path. This router then responds with an ICMP Time Exceeded Message (TEM) indicating that the datagram has expired.
Three more UDP messages are now sent, this time with the TTL value set to 2. This causes the second router to return ICMP TEMs. This process continues until the packets actually reach the other destination or the maximum TTL has been reached. The default maximum TTL for traceroute is 30.
Since these datagrams are trying to access an invalid port at the destination host, ICMP Port Unreachable Messages are returned instead of ICMP TEMs. This indicates an unreachable port and signals the Traceroute program to end the process.
The Lab Activity will help students become more familiar with the traceroute command.
The next page will discuss some other commands that are used to troubleshoot routing issues.
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