Understanding EIGRP

Hello learners! 

We all know that routing is the process of guiding your data packet from its source to its destination, using routers along the way. Sounds simple, right? Well, in this session, we’re diving deeper into one of the most powerful routing protocols out there: Enhanced Interior Gateway Routing Protocol (EIGRP).In this article, we'll explore EIGRP's core concepts, including: 

EIGRP Best path operation, the table it maintains and important EIGRP Terminologies .So, stay with me till the end, I promise you'll walk away with a solid understanding of how EIGRP works and why it's important. 

You must be thinking why EIGRP was crated? Right? 

If you’ve ever heard of IGRP (Interior Gateway Routing Protocol), you might know it was one of Cisco’s earlier routing protocols. But it had a few limitations: 

• It was classful, meaning it didn’t support Variable Length Subnet Masking (VLSM) 

• It lacked scalability 

• It wasn’t very efficient for growing networks 

To overcome these issues, Cisco developed EIGRP, a protocol that enhanced IGRP by adding more advanced features and flexibility. Originally, EIGRP was a Cisco proprietary protocol, meaning it only worked on Cisco devices. But that changed in May 2016, when Cisco released EIGRP to the IETF through RFC 7868, making it an open standard. That means today, other vendors can also implement EIGRP in their systems. 

The Fundamentals of EIGRP 

EIGRP does overcome the deficiencies of other distance vector routing protocols, such as Routing Information Protocol (RIP) with many features. Let’s see the key features of it. 

• Advanced Distance Vector Intelligence: EIGRP is more than a basic distance vector protocol, it's smart. It calculates the best path not just based on hop count, but using: Interface Bandwidth, Delay, Reliability, Load, MTU Size. This results in more accurate and performance-oriented routing decisions. 

• Supports Variable Length Subnet Mask (VLSM): EIGRP is classless, which means it supports VLSM, allowing you to create smaller, more efficient subnets and build more flexible routing tables. 

• Unequal-Cost Load Balancing: Why waste a good backup link? EIGRP can distribute data traffic across multiple paths with different costs, making the most of all available bandwidth. 

• Rapid Convergence with DUAL: Thanks to the Diffusing Update Algorithm (DUAL), EIGRP can precompute loop-free backup paths. When a primary route fails, the backup kicks in almost instantly, keeping your network stable and responsive. 

EIGRP Terminologies: Understand the language of EIGRP 

To truly understand how EIGRP works, it's important to get familiar with some key terms. These help you interpret routing decisions and troubleshoot EIGRP behavior more effectively. Here’s a breakdown of the most essential EIGRP terminologies: 

• Successor Route: This is the best route to reach a destination, the one with the lowest metric (called the feasible distance). It’s the route EIGRP installs in the routing table. 

• Successor: The next-hop router that EIGRP uses to forward traffic along the successor route. 

• Feasible Successor: A backup route that meets the feasibility condition. It’s kept in the topology table and is ready to be used if the primary route fails. 

• Feasible Distance (FD): The total metric (cost) to reach a destination via the successor route. It includes the cost from the local router to the destination. 

• Reported Distance (RD): Also known as the advertised distance, this is the metric value reported by a neighbor (your successor) to reach the destination. 

• Feasibility Condition (FC): For a route to qualify as a feasible successor, its reported distance (RD) must be less than the feasible distance (FD) of the current successor. This ensures loop-free routing. 

Feeling overwhelmed? Don’t worry some of these terms might seem a bit tricky at first. But don’t stress, we’ll make it simple! Let’s understand all of this better with the help of the diagram below: 

Understanding EIGRP Path Selection – Explained with a Diagram 

Learners, take a good look at the diagram above. We're going to understand how EIGRP uses its DUAL algorithm to select the best path (successor) and a loop-free backup path (feasible successor). 

Step-by-Step Walkthrough: 

Let's focus on R1, which wants to reach the destination network 10.4.4.0/24. 

R3 is advertising a Reported Distance (RD) of 15 to R1 (calculated as 5 + 10). 

R1 adds its own cost and gets a Feasible Distance (FD) of 20 via R3. 

Since this is the lowest metric, R3 becomes the Successor Router, and its route is selected as the Successor Route. 

Successor Route: R1 → R3 → 10.4.4.0/24.  

Now let’s look at backup paths: 

1. R2 advertises an RD of 20, which is equal to R1’s FD (20) — so it does not meet the feasibility condition. 

1. R4 advertises an RD of 15, which is less than R1’s FD (20) — so it meets the feasibility condition! 

That means R4 becomes the Feasible Successor, ready to take over if the primary path through R3 fails. 

Let’s see what is the condition for backup path:  

Feasible condition: Reported distance of the feasible successor < feasible distance    of successor 

Let’s understand the reported distance and feasible distance with the help of table: 

See? That wasn’t tough at all! Once you understand the concept of RD, FD, and the feasibility condition, it all starts to make sense. Sounds good, right?  

EIGRP Tables: 

In EIGRP, routing decisions don’t just happen randomly — there’s a well-structured process behind it. To establish neighbor relationships, learn about available paths, and choose the best route. EIGRP uses three important tables. 

1. EIGRP Neighbor Table: This is the first table built when EIGRP is enabled on a router interface. It stores information about directly connected routers that have successfully formed an EIGRP adjacency. 

• Neighbor IP addresses 

• Interface used 

• Hold time (to detect failure)  

Think of it as your router’s contact list. 

 2. EIGRP Topology Table: Once neighbors are established, routers begin exchanging route information. All of that data gets stored here, even the backup routes! What it includes: 

• All routes learned from neighbors 

• Metrics (bandwidth, delay, etc.) 

• Feasible Successors and Successors 

This table is like your router’s knowledge base of all available paths. 

3. EIGRP Routing Table: Only the best paths (i.e., the lowest-metric, loop-free routes) from the topology table are promoted to the routing table. These are the paths your router actually uses to forward packets. It includes: 

• Best (Successor) routes only 

This is the router’s final decision, the actual path data takes. 

And there you have it, learners!  We’ve explored the essentials of EIGRP, from its intelligent route calculation and rapid convergence to its powerful use of tables and terminology. 

Next time you're configuring or troubleshooting a network, remember that EIGRP does more than just find a path, it intelligently finds the best path. 

Learners! Do you have questions or want to share your lab experience? Drop a comment or reach out, let’s grow together as network engineers! 

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