EIGRP Explained | Step by Step

Introduction

This tutorial will guide you through the fundamentals of Enhanced Interior Gateway Routing Protocol (EIGRP), a Cisco proprietary routing protocol used within a single autonomous system. EIGRP combines features of both distance vector and link-state protocols, making it a robust choice for efficient routing. Understanding EIGRP is essential for network professionals, particularly those preparing for the CCNA certification.

Step 1: Establishing Neighbour Relationships

To begin using EIGRP, routers must first establish neighbour relationships. This is crucial for enabling communication and sharing routing information.

• Ensure EIGRP is enabled on the routers.
• Use the following command to enable EIGRP on a router:

  router eigrp [AS_number]
  

  Replace [AS_number] with your autonomous system number.
• Verify neighbour relationships with the command:

  show ip eigrp neighbors
  

• Check for successful adjacency, which means the routers can communicate with each other.

Step 2: Exchanging Routing Information

Once neighbours are established, the routers will exchange routing information.

• Initial communication involves sending full updates, which include the complete routing table.
• After the first exchange, routers send partial updates only when changes occur, reducing unnecessary traffic.
• Use the command:

  show ip route eigrp
  

  to view the routes learned through EIGRP.

Step 3: Choosing the Best Routes

After exchanging information, each router must determine the best routes to include in its routing table.

• EIGRP uses a metric based on bandwidth, delay, load, and reliability to evaluate routes.
• The formula for calculating the EIGRP metric is:

  Metric = (K1 * Bandwidth + K2 * Bandwidth / (256 - Load) + K3 * Delay) * 256
  

• Ensure route metrics are configured properly to optimize performance.
• Use the command:

  show ip eigrp topology
  

  to see the topology table and the routes chosen by EIGRP.

Practical Tips

• Always keep your EIGRP configurations consistent across routers in the same autonomous system.
• Monitor EIGRP performance regularly using commands like show ip eigrp statistics to identify any issues.
• Be cautious of potential pitfalls, such as mismatched EIGRP configurations, which can prevent routers from forming neighbour relationships.

Conclusion

In this tutorial, you learned the essential steps to configure and understand EIGRP, from establishing neighbour relationships to exchanging routing information and selecting optimal routes. Mastering EIGRP is vital for effective network routing, especially for those pursuing networking certifications. For further learning, consider exploring additional resources or practice exams related to EIGRP and CCNA topics.