What is EIGRP?

Welcome to EIGRP.guru!

This comprehensive tutorial will take you from zero EIGRP knowledge to expert-level understanding. We'll cover everything from basic concepts to DUAL algorithm analysis, complete with configuration examples and interactive tools.

Enhanced Interior Gateway Routing Protocol (EIGRP) Overview

EIGRP is a hybrid routing protocol that combines the advantages of distance vector and link-state protocols. It uses the Diffusing Update Algorithm (DUAL) to ensure loop-free routing and fast convergence.

Key Characteristics

  • Hybrid Protocol: Combines distance vector and link-state features
  • Fast Convergence: Uses DUAL algorithm for rapid route computation
  • Loop-Free Routing: Guaranteed loop-free paths
  • Unequal Cost Load Balancing: Supports traffic distribution across multiple paths
  • Classless Routing: Supports VLSM and CIDR
  • Incremental Updates: Only sends changes, not full routing tables

EIGRP vs Other Routing Protocols

Aspect EIGRP OSPF RIP
Algorithm DUAL Dijkstra SPF Bellman-Ford
Convergence Speed Very Fast Fast Slow
CPU Usage Low High during SPF Low
Memory Usage Medium High Low
Scalability Good Excellent Poor
Vendor Support Cisco (now open) Multi-vendor Multi-vendor

Why Choose EIGRP?

EIGRP is ideal for networks that need:

  • Fast convergence with minimal network overhead
  • Unequal cost load balancing capabilities
  • Simple configuration and management
  • Excellent scalability in medium to large networks
  • Support for multiple network layer protocols

The EIGRP Process

Neighbor Discovery
→
Topology Exchange
→
Route Computation
→
Route Installation

The EIGRP process involves four main phases:

  1. Neighbor Discovery: Routers discover neighbors using Hello packets
  2. Topology Exchange: Routers exchange topology information
  3. Route Computation: DUAL algorithm computes best paths
  4. Route Installation: Best routes are installed in the routing table

Quick Start Example

Here's a basic EIGRP configuration to get you started:

# Cisco Configuration
router eigrp 100
 network 192.168.1.0 0.0.0.255
 network 10.0.0.0 0.0.0.255
 no auto-summary

History & Evolution

From IGRP to EIGRP

EIGRP evolved from Cisco's Interior Gateway Routing Protocol (IGRP) to address the limitations of classful routing and improve convergence times in large networks.

Timeline of Development

Year Protocol Key Features Limitations
1985 IGRP Composite metric, 15-second updates Classful routing, slow convergence
1992 EIGRP DUAL algorithm, fast convergence Cisco proprietary
1994 EIGRP for IP VLSM support, classless routing IPv4 only
1996 EIGRP for IPX Multi-protocol support Protocol-dependent modules
1998 EIGRP for IPv6 IPv6 support Separate processes
2013 EIGRP RFC 7868 Open standard Limited adoption

Key Improvements Over IGRP

EIGRP Advantages

  • Fast Convergence: Sub-second convergence times
  • Loop-Free Routing: DUAL algorithm prevents loops
  • Unequal Cost Load Balancing: Traffic distribution
  • Classless Routing: VLSM and CIDR support
  • Incremental Updates: Bandwidth efficient
  • Neighbor Authentication: Security features

IGRP Limitations

  • Slow Convergence: 90-second hold-down timers
  • Routing Loops: Susceptible to loops
  • Equal Cost Only: No unequal cost load balancing
  • Classful Routing: No VLSM support
  • Periodic Updates: Every 90 seconds
  • No Authentication: Security concerns

The DUAL Algorithm Innovation

Revolutionary Algorithm

The Diffusing Update Algorithm (DUAL) was the key innovation that made EIGRP possible:

  • Loop-Free Paths: Mathematically guaranteed loop-free routing
  • Fast Convergence: Immediate route computation when topology changes
  • Feasible Successors: Backup routes computed in advance
  • Query Process: Distributed computation for complex scenarios

RFC 7868 - EIGRP as an Open Standard

In 2013, Cisco published EIGRP as an open standard in RFC 7868, making it available for implementation by other vendors.

Aspect Before RFC 7868 After RFC 7868
Vendor Support Cisco only Open for all vendors
Implementation Proprietary Standardized
Interoperability Cisco devices only Multi-vendor support
Adoption Limited to Cisco networks Broader industry adoption

Current Status

Despite being an open standard, EIGRP adoption outside Cisco environments remains limited. Most vendors and organizations continue to prefer OSPF for multi-vendor deployments.

EIGRP vs Other Protocols

Routing Protocol Classification

Understanding how EIGRP compares to other routing protocols helps in making informed network design decisions.

Protocol Categories

Category Protocol Algorithm Convergence Scalability
Distance Vector RIP Bellman-Ford Slow Poor
EIGRP DUAL Very Fast Good
Link State OSPF Dijkstra Fast Excellent
IS-IS Dijkstra Fast Excellent
Path Vector BGP Path Attributes Slow Excellent

When to Choose EIGRP

EIGRP is Ideal For:

  • Cisco-Only Networks: Homogeneous Cisco environment
  • Fast Convergence Requirements: Real-time applications
  • Simple Configuration: Limited network administration resources
  • Unequal Cost Load Balancing: Traffic engineering needs
  • Hub-and-Spoke Topologies: With stub routing

Key Terminology

Understanding EIGRP terminology is essential for mastering the protocol. This section covers all the key terms and concepts you'll encounter.

Core EIGRP Terms

Fundamental Concepts

Term Definition Significance
DUAL Diffusing Update Algorithm Core algorithm ensuring loop-free routing
Feasible Distance (FD) Best metric to reach a destination Determines the successor route
Reported Distance (RD) Neighbor's metric to reach a destination Used in feasibility condition
Successor Next-hop router for the best path Primary route in routing table
Feasible Successor Backup route that meets feasibility condition Pre-computed backup path

Packet Types

Packet Type Opcode Purpose Reliability
Hello 5 Neighbor discovery and keepalive Unreliable
Update 1 Routing information exchange Reliable
Query 3 Route recomputation request Reliable
Reply 4 Response to Query Reliable
ACK 5 Acknowledgment Unreliable

Memory Aid

Remember EIGRP packet types with the mnemonic: "Happy Users Query Routers Accordingly" (Hello, Update, Query, Reply, ACK)

Next: Chapter 2 - EIGRP Fundamentals