1.1.3 Troubleshooting with Layered Models
The OSI and TCP/IP models can be applied to isolate network problems when troubleshooting. For example, if the symptoms suggest a physical connection problem, the network technician can focus on troubleshooting the cables and their connections at the physical layer.
The figure shows some common devices and the OSI layers that must be examined during the troubleshooting process for that device.
Notice that routers and multilayer switches are shown at Layer 4, the transport layer. Although routers and multilayer switches usually make forwarding decisions at Layer 3, ACLs on these devices can be used to make filtering decisions using Layer 4 information.