What is Network Troubleshooting: How to Test Local Network

By | 9th November 2015

Testing a host on the local LAN.

After Successfully pinging remote hosts, both the local host -the router – and the remote host are configured correctly. Pinging each host one by one on the LAN can carry out this test.

If a host responds with Destination Unreachable, note which address was not successful and continue to ping the other hosts on the LAN.

Another failure message is Request Timed Out. This indicates that no response was made to the ping attempt in the default time period indicating that network latency may be an issue.

Testing a host

The IOS offers and extended mode of the ping command. This mode is entered by typing ping in privileged EXEC mode, at the CLI prompt without assigning a destination IP address. A series of prompts are then presented as shown in this example. Pressing Enter accepts the indicated default values.

Router#ping
Protocol [ip]:
Target IP address:10.0.0.1
Repeat count [5]:
Datagram size [100]:
Timeout in seconds [2]:5
Extended commands [n]: n




Entering a longer timeout period than the default allows for possible latency issues to be detected. If the ping test is successful with a longer value, a connection exists between the hosts, but latency may be an issue on the network.

Note that entering “y” to the “Extended commands” prompt provides more options that are useful in troubleshooting.

A Successfully ping shows that the local and other hosts IP address in the network are configured properly.

testing local network

Testing Gateway and Remote Connectivity

The next step is to test if the local host can connect with a gateway address.

 You can Use ping command to verify if the local host can connect the gateway.  This is extremely important because the gateway is the host’s entry and exit to the wider network. If the ping command returns a successful response, connectivity to the gateway is verified.




To begin, choose a station as the source device. In this case, we chose 192.168.1.1 as shown in the figure above to be the gateway IP address.

c:>ping 192.168.1.1

The gateway IPv4 address should be readily available in the network documentation, but if it is not available, use the ipconfig command to discover the gateway IP address.

If the gateway test fails:

1.         Try pinging another host in the local LAN to verify that the problem is not the source host.

2.         Then verify the gateway address with the network administrator to ensure that the proper   address is being verified

If all devices are configured properly, check the physical cabling to ensure that it is secure and properly connected. Keep an accurate record of what attempts have been made to verify connectivity. This will assist in solving this problem and, perhaps, future problems.

Testing Route Next Hop

In a router, you can use IOS to test the next hop of the individual routes. Each route has the next hop listed in the routing table. You can use the output of the show ip route command to determine the next hop. Frames carrying packets that are directed to the destination network listed in the routing table are sent to the device that represents the next hop. If the next hop is not accessible, the packet will be dropped.

To test the next hop, determine the appropriate route to the destination and try to ping the appropriate next hop for that route in the routing table. A failed ping indicates that there might be a configuration or hardware problem.

 The ping may also be prohibited by security in the device. If the ping is successful you can move on to testing connectivity to remote hosts.

Testing Remote Hosts connectivity

testing remote connectivity




Once verification of the local LAN and gateway is complete, testing can proceed to remote devices, which is the next step in the testing process.

The figure depicts a sample network topology. There are 3 hosts within a LAN, a router (acting as the gateway) that is connected to another router (acting as the gateway for a remote LAN), and 3 remote hosts. The verification tests should begin within the local network and progress outward to the remote devices.

                                   Ping a remote host from a local host

Begin by testing the outside interface of a router that is directly connected to a remote network. In this case, the ping command is testing the connection to 200.10.10.129, the outside interface of the local network gateway router.

If the ping command is successful, connectivity to the outside interface is verified. Next, ping the outside IP address of the remote router, in this case, 200.10.10.130 If successful, connectivity to the remote router is verified. If there is a failure, try to isolate the problem. Retest until there is a valid connection to a device and double-check all addresses.

The ping command will not always help with identifying the underlying cause to a problem, but it can isolate problems and give direction to the troubleshooting process. Document every test, the devices involved, and the results.

Test Router Remote Connectivity

A router forms a connection between networks by forwarding packets between them. To forward packets between any two networks, the router must be able to communicate with both the source and the destination networks. The router will need routes to both networks in its routing table.

To test the communication to the remote network, you can ping a known host on this remote network. If you cannot successfully ping the host on the remote network from a router, you should first check the routing table for an appropriate route to reach the remote network. It may be that the router uses the default route to reach a destination. If there is no route to reach this network, you will need to identify why the route does not exist. As always, you also must rule out that the ping is not administratively prohibited.




Networking Troubleshootinh: Tracing and Interpreting Results

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