EIGRP Protocol: Complete Guide

How to Study EIGRP Using This Lab Hub

This hub gives you a simple path for learning Enhanced Interior Gateway Routing Protocol. Start with neighbor discovery and the three EIGRP tables. Then learn metrics, DUAL, configuration, and troubleshooting.

Do not memorize commands alone. In a useful lab, you should be able to explain why a neighbor formed, why one path became the successor, whether a backup path qualifies as a feasible successor, and why a route entered Active state.

EIGRP Quick Summary

EIGRP is a classless routing protocol designed for internal routing within an autonomous system. It discovers directly connected neighbors with Hello packets, stores learned paths in a topology table, uses DUAL to select loop-free routes, and sends partial triggered updates when the topology changes.

The composite metric uses bandwidth and delay by default. EIGRP can install equal-cost paths and supports unequal-cost load balancing through the variance feature. EIGRP supports IPv4 and IPv6 and is formally documented in RFC 7868.

Recommended EIGRP Study Order

1. Begin with the EIGRP overview to understand protocol 88, neighbor discovery, DUAL, and classless routing.
2. Review EIGRP packet types so Hello, Update, Query, Reply, and Acknowledgement behavior is clear.
3. Study EIGRP metrics and K-values before changing bandwidth, delay, or variance.
4. Practice equal and unequal-cost load balancing and prove which paths can be installed.
5. Practice EIGRP configuration with network statements, passive interfaces, summarization, and verification commands.
6. Compare IPv4, IPv6, global, and VRF address families in named mode.
7. Use EIGRP stub routing to control branch advertisements and contain Queries.
8. Use the DUAL algorithm module to understand successors, feasible successors, feasibility condition, and Active routes.
9. Finish with EIGRP troubleshooting to diagnose neighbor failures, missing routes, and Stuck-in-Active conditions.

Before You Start an EIGRP Lab

Write down the autonomous system number, router IDs, interface addresses, advertised networks, passive interfaces, expected neighbors, link bandwidth and delay, and the routes you expect to install.

Keep default K-values unless the lab has a clear reason to change them. Record the working output before introducing a fault so you can compare the before and after states.

EIGRP Protocol Architecture

EIGRP routers discover directly connected neighbors by sending Hello packets. When neighbor parameters are compatible, the routers establish an adjacency and exchange routing information.

Received route information is placed in the EIGRP topology table. DUAL evaluates available paths, selects a successor, identifies qualifying feasible successors, and submits the best route to the routing table.

After the initial exchange, EIGRP does not periodically advertise the complete routing table. It sends partial, bounded, triggered updates only when routing information changes.

Neighbor, Topology and Routing Tables

The neighbor table records directly connected EIGRP peers, interfaces, hold times, sequence information, and reliable-delivery statistics. The topology table stores learned paths, reported distances, feasible distances, successors, feasible successors, and Active or Passive route state.

The routing table contains the selected EIGRP successor route when it wins route installation. Internal EIGRP routes normally appear with the code D, while redistributed external EIGRP routes normally appear as D EX.

DUAL and Feasibility

DUAL is the central EIGRP path-calculation and convergence mechanism. A successor is the lowest-metric loop-free path selected for the routing table. A feasible successor is a loop-free backup path that satisfies the feasibility condition.

When the successor fails, DUAL first checks the topology table for a feasible successor. If one exists, EIGRP can install it immediately. If no feasible successor exists, the route enters Active state and the router sends Query packets to neighbors.

The EIGRP stub feature reduces query scope and helps prevent large query domains, excessive convergence delays, and Stuck-in-Active conditions.

Timers, Multicast and Packet Types

EIGRP uses multicast address 224.0.0.10 for IPv4 and FF02::A for IPv6. Common Hello and hold timers are 5 seconds and 15 seconds on most interfaces, while low-speed NBMA links commonly use 60 seconds and 180 seconds.

Hello packets discover and maintain neighbors. Updates advertise changed routing information. Queries ask neighbors for loop-free paths, Replies answer those queries, and Acknowledgements confirm receipt of reliable packets.

Unlike OSPF, EIGRP Hello and hold timers do not have to match for an adjacency to form. K-values must match because neighbors must use the same metric calculation.

EIGRP Design and Troubleshooting Notes

Configure a stable router ID, keep default K-values unless there is a documented design requirement, and use bandwidth and delay carefully because they influence route selection.

Use summarization, stub routing, filtering, and sound hierarchy to limit query scope. Remember that unequal-cost load balancing with variance still requires loop-free qualifying routes.

Useful verification commands include show ip eigrp neighbors, show ip eigrp interfaces detail, show ip eigrp topology, show ip protocols, and show ip route eigrp.

What to Verify in Every EIGRP Lab

• Use show ip eigrp neighbors to confirm the peer, interface, hold time, and reliable-delivery statistics.
• Use show ip eigrp interfaces detail to confirm EIGRP is active on the correct links.
• Use show ip eigrp topology to identify successors, feasible distance, reported distance, and route state.
• Use show ip route eigrp to confirm the selected internal and external routes.
• Use a sourced ping and traceroute to prove both forward and return paths.

Practical EIGRP Lab Method

Build a three-router triangle with one higher-cost link. Confirm all neighbors, identify the successor, and check whether the alternate path meets the feasibility condition. Then fail the successor link and observe whether convergence is immediate or the route enters Active state.

After restoring the baseline, introduce one fault at a time: a different autonomous system number, mismatched K-values, a passive interface, an incorrect network statement, a route filter, or a missing reply to an EIGRP Query. For each fault, record the first command that reveals the cause.

Related Routing and Certification Resources

• OSPF Labs for link-state routing comparison.
• RIP Labs for distance-vector fundamentals and timer behavior.
• BGP Labs for policy-driven path-vector routing.
• Routing Practice Test for EIGRP, OSPF, BGP, RIP, metrics, and path selection.

EIGRP Lab Frequently Asked Questions

What should I learn first in EIGRP?

Start with Hello packets, neighbor formation, the neighbor table, topology table, and routing table. Then study the metric, successor selection, feasible successors, and DUAL.

What is the difference between a successor and a feasible successor?

The successor is the best loop-free path installed in the routing table. A feasible successor is a verified loop-free backup path that satisfies the feasibility condition and can be used quickly if the successor fails.

Do EIGRP Hello and hold timers need to match?

No. Unlike OSPF Hello and dead timers, EIGRP Hello and hold timers do not have to match. Each router tells its neighbor how long to keep the relationship alive.

Why is an EIGRP route Stuck in Active?

A route can become Stuck in Active when a router sends Queries but does not receive every required Reply before the Active timer expires. Check failed links, overloaded routers, large query domains, and missing stub or summarization boundaries.

Which EIGRP settings must match between neighbors?

Neighbors must use the same autonomous system number and K-values, and authentication must agree when enabled. They also need compatible addressing and working connectivity on the shared link.