How to use the OSI Model to Troubleshoot Networks

When a get a call from a user that explains a problem they are having, I would immediately visualize the OSI model. The user might say that they can’t bring up a graphic they are trying to download from the Internet. That graphic is brought up in an application.  (layer 7 of the OSI model). I could either start at the top or the bottom, depending on what I suspected was the problem. I would usually start at the bottom (termed the “bottom up approach”). At the bottom of the OSI model is the physical layer (layer 1). So, I would proceed to ask them questions like this:

• Is your network cable plugged in? (physical)
• Is there a link light on the Ethernet switch and Ethernet NIC? (data-link)
• Do you have an IP address? (network)
• Can you ping your default gateway? (network, testing LAN IP connectivity)
• Do you have DNS server information?
• Can you ping your DNS server? (network, testing IP connectivity)
• Do you have a firewall configured? (network on up to application)
• Can you ping the host you are trying to get to by name? (application, DNS and network WAN IP connectivity)
• What format is the graphic in? Do you have a viewer for that format? (presentation)
• Can your web browser open up another website? (basic application troubleshooting) 

How to use the OSI Model to Troubleshoot Networks
by David Davis, vExpert, VCP, CCIE 9369 - January 7, 2009
http://www.petri.co.il/csc_how_to_use_the_osi_model_to_troubleshoot_networks.htm