Cisco Certified Network Associate (CCNA) 2026 – 400 Free Practice Questions to Pass the Exam

A serial interface status of 'status up/protocol down' indicates that the physical layer of the OSI model is functioning correctly, meaning that there is an established connection and the interface is capable of sending and receiving bits. However, the 'protocol down' condition signifies that the data link layer is having issues with establishing a proper connection for communication with the devices at Layer 2. This can occur for several reasons, such as misconfigured encapsulation, line protocol mismatches, or issues with the framing.

In this context, the 'status up' suggests that the physical connection (Layer 1) is active, but since the protocol is down, it indicates a problem at the data link layer where the interface fails to establish a logical connection required for data exchange. Thus, the proper identification of this status reflects the data link layer’s functionality and health in supporting communication.

Understanding this will help drive home the concept of how the OSI model layers interact, particularly how a reliable physical link doesn’t ensure data link layer functionality, which is critical for effective network communication.