|  | 4.4 | Basic Networking Testing
| 4.4.6 |
Using the show interfaces serial command to test the physical and data link layers
As shown in Figure , the interface has two pieces, physical (hardware) and logical (software): The hardware -- such as cables, connectors, and interfaces -- must make the actual connection between the devices. The software is the messages -- such as keepalive messages, control information, and user information -- that are passed between adjacent devices. This information is data being passed between two connected router interfaces. When you test the physical and data link, you ask these questions: Is there a Carrier Detect signal? Is the physical link between devices good? Are the keepalive messages being received? Can data packets be sent across the physical link? One of the most important elements of the show interfaces serial command output is display of the line and data link protocol status. Figure indicates the key summary line to check the status meanings. The line status in this example is triggered by a Carrier Detect signal, and refers to the physical layer status. However, the line protocol, triggered by keepalive frames, refers to the data link framing.