Examine the figure and answer the question. (Click the image to enlarge)
(see answer below and video response.)
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The best answer is C. Session.
Explanation: If VPN connections cannot be made, the problem can often be considered to reside on the session layer of the OSI model. This is one of the reasons that the OSI model exists; to help a technician understand the inner workings of a particular technology (or protocol) and troubleshoot any errors that might arise. In this scenario, connections can not be made. This is often due to authentication and authorization problems, which happen at the session layer, as does session restoration. For example, a Microsoft #691 error (which means the username/password failed and/or authentication protocol is not permitted). Here’s a link to many of the Microsoft VPN error codes: https://blogs.technet.microsoft.com/rrasblog/2009/08/12/troubleshooting-common-vpn-related-errors/
Incorrect Answers. Physical layer errors deal with tangible objects such as cabling, ports, plugs, transceivers, and so on. Network layer issues concern things such as IP address configuration problems. An application layer problem could mean that the VPN is functional, but DNS does not work properly, and so name resolution over the VPN connection fails. There are many errors that can occur with VPN connections (such as PPTP and L2TP). These often will display error codes which, when used with the OSI or TCP/IP models, can help you to troubleshoot the actual problem. For more about the OSI model, see my video here: http://www.davidlprowse.com/articles/?p=905.
(Answer at 0:42)