Cisco Networking: Network and Transport Layer Troubleshooting
Cisco network administrators must be familiar with the seven layers of the OSI model, two of which the network and transport layers. A myriad of problems can occur at the network and transport layers; here are some quick troubleshooting tips:
- Check your protocol address: If you use an incorrect protocol address for your network, you cannot communicate with any devices on your network.
Attempt to communicate with your router: If you use a routable protocol, such as IP, then you should attempt to communicate with your router, or a device on the other side of your router (such as a server on the Internet). Ping is a great tool for testing this connectivity. It will give you instant feedback if you can communicate with the other device.
Attempt to communicate with other devices on your network segment or data link: Your router from the previous item is a device on your data link that should always be up and running. If you could not communicate with the router, try another device, such as another computer, because it could be that your router is not working properly.
Most of the issues that occur at these layers are typically resolved with the configuration of the network protocol on your device or another device on the network.
If you use IP as your network protocol, something as simple as entering the incorrect subnet mask on a device can cause you to not be able to communicate with a large number of other network devices, either on your data link or on the other side of a router.
Other issues that occur at these layers revolve around name resolution. For example, you may be able to get to the destination server by IP, but if the name you are resolving is wrong, you cannot connect. Alternatively, you could be resolving the correct name but are getting the wrong address in your resolution.