Cisco Certified Network Associate (CCNA) Practice Exam

Which of the following protocols is NOT a commonly used transport layer protocol?

1. UDP

2. TCP

3. IP

4. GRE

The correct answer is: IP

The selection of this answer is based on the understanding of the OSI model, specifically the roles of different layers. The transport layer, which is responsible for end-to-end communication and error recovery between devices, includes protocols like UDP (User Datagram Protocol) and TCP (Transmission Control Protocol). Both are widely recognized for ensuring reliable or unreliable communication over networks, handling flow control, and managing data segmentation. On the other hand, IP (Internet Protocol) functions at the network layer, which is responsible for routing packets across network boundaries and tackling logical addressing. It is primarily concerned with the delivery of packets, rather than the transportation of data streams as seen in transport layer protocols. GRE (Generic Routing Encapsulation) is a tunneling protocol used to encapsulate a wide variety of network layer protocols into a point-to-point connection. While it operates at a different level (primarily Layer 3), GRE can also contribute to transport-like functions, but it is not classified as a transport layer protocol in the context of the OSI model. Thus, identifying IP as a protocol that does not belong to the transport layer is accurate, as it serves a different purpose within the network architecture.