At which layer of the OSI model are routing protocols defined?

The Network Layer (Layer 3) of the OSI model is responsible for packet forwarding, including routing through intermediate routers. It manages the logical addressing of devices (e.g., IP addresses) and determines the optimal path for data to travel from a source to a destination across different networks. Routing protocols, such as Open Shortest Path First (OSPF) and Border Gateway Protocol (BGP), operate at this layer to exchange reachability information among routers, enabling them to build and maintain routing tables. This path determination function is a core responsibility of the Network Layer.

B. Physical: This layer is responsible for the transmission and reception of unstructured raw data between a device and a physical transmission medium.
C. Transport: This layer provides end-to-end communication services, managing data segmentation and reassembly, but it relies on the Network Layer for routing.
D. Data Link: This layer handles node-to-node data transfer and physical addressing (MAC addresses) within a single local network segment, not inter-network routing.

1. Palo Alto Networks. (2020). Cybersecurity Survival Guide: A Palo Alto Networks Core Learning Resource. Chapter 3, "The Connected World," Section: "The OSI Model," p. 49. The guide states, "The network layer (Layer 3) is responsible for routing traffic between networks."

2. Kurose, J. F., & Ross, K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. Chapter 5, "The Network Layer: Control Plane," Section 5.1, "Introduction," p. 426. This university-level textbook explicitly defines the network layer's control plane as being responsible for routing, stating, "The main role of the network layer’s control plane is routing... A routing protocol is said to run in each and every router."

3. Stanford University. (2021). CS144: Introduction to Computer Networking, Lecture 1 Slides. Slide 33, "Layer 3: Network Layer." The course material specifies the key functions of the Network Layer as "Routing packets from source to destination" and "Making routing decisions."