This chapter discusses key troubleshooting concepts that aid in resolving network issues. These concepts include maintaining up-to-date documentation and how to systematically isolate network issues and correct them. We start with the physical medium, as many engineers simply ignore this layer, but you will be shown why it is important. The chapter provides examples and steps that can be used to resolve issues dealing with VLANs, trunking, EtherChannels, VTP, STP, static routing, RIP, EIGRP, OSPF, and BGP. There are plenty of exercises at the end of the chapter to reinforce what you have learned.