EIGRP operates at Layer 3 of the OSI model.
Enhanced Interior Gateway Routing Protocol (EIGRP) is a widely used dynamic routing protocol primarily developed by Cisco. Its fundamental function is to enable routers to exchange routing information and select optimal paths for data packets to travel across networks.
EIGRP and the OSI Model
The Open Systems Interconnection (OSI) model is a conceptual framework used to describe the functions of a networking system. It divides networking communication into seven layers. EIGRP's operations align specifically with the network layer.
- Layer 3: Network Layer
- This layer is responsible for logical addressing (like IP addresses) and routing.
- Protocols at this layer determine the best path for data packets to travel from the source network to the destination network across different interconnected networks.
- Routing protocols like EIGRP function at this layer to build and maintain routing tables.
As highlighted by the reference from nwkings.com, "EIGRP in networking... works on layer 3 of the OSI model and helps find the best path." This confirms EIGRP's classification at the Network Layer, where it performs its core function of route discovery and selection.
Why EIGRP is a Layer 3 Protocol
EIGRP's role is inherently tied to the network layer because it deals directly with:
- Network Addresses: It uses IP addresses to identify networks and hosts.
- Routing: It determines the best path or route for data packets to traverse between networks.
- Packet Forwarding: Although actual packet forwarding is done by the data plane, the routing decisions made by EIGRP at Layer 3 dictate how packets are forwarded.
Unlike protocols at higher layers that deal with application data or session management, or lower layers that handle physical transmission or data link framing, EIGRP's focus is on the logical addressing and routing infrastructure necessary to move data across different networks.
OSI Model Layers
Here's a simplified view of the OSI model, indicating where EIGRP fits:
| OSI Layer | Function | Examples |
|---|---|---|
| 7. Application | Network process to application | HTTP, FTP, SMTP |
| 6. Presentation | Data representation, encryption, decryption | JPEG, MPEG, SSL/TLS |
| 5. Session | Interhost communication, managing sessions | NetBIOS, RPC |
| 4. Transport | End-to-end connections, reliable data transfer | TCP, UDP |
| 3. Network | Routing, logical addressing | IP, ICMP, EIGRP, OSPF, RIP |
| 2. Data Link | Physical addressing, error handling | Ethernet, PPPoE, Frame Relay |
| 1. Physical | Physical medium, binary transmission | Cables, Connectors, Bits |
As shown, EIGRP resides firmly within the Network Layer (Layer 3).
Evolution of EIGRP
The reference also notes that EIGRP is "an updated version of the IGRP protocol." IGRP (Interior Gateway Routing Protocol) was an earlier routing protocol also developed by Cisco, and it too operated at the Network Layer. EIGRP built upon IGRP, introducing more advanced features like partial route updates, bounded updates, unequal-cost load balancing, and the use of the Diffusing Update Algorithm (DUAL) for rapid convergence, significantly improving its efficiency and scalability compared to its predecessor.
Understanding EIGRP's placement at Layer 3 is crucial for comprehending how it interacts with other network protocols and how routing decisions are made within an IP network infrastructure.
