EIGRP

EIGRP (Enhanced Interior Gateway Protocol) is an advanced distance-vector routing protocol developed by Cisco Systems. It combines the advantages of distance-vector protocols with the loop-prevention mechanisms of link-state protocols, offering fast convergence and efficient bandwidth usage.

Overview

EIGRP is a Cisco-proprietary hybrid routing protocol that uses the Diffusing Update Algorithm (DUAL) to provide loop-free routing and fast convergence. It's designed to be more efficient than traditional distance-vector protocols while maintaining simplicity in configuration and operation.

Historical Context

Development

1990s: EIGRP developed by Cisco as successor to IGRP
1993: EIGRP officially released
2013: EIGRP released as open standard (RFC 7868)
Present: Still widely used in Cisco environments

Evolution from IGRP

IGRP: Original Cisco distance-vector protocol
EIGRP: Enhanced version with better features
Improvements: Faster convergence, VLSM support
Compatibility: Migration path from IGRP

EIGRP Architecture

Components

EIGRP Tables

Neighbor Table: List of adjacent routers
Topology Table: All learned routes (successors and feasible successors)
Routing Table: Best routes installed for forwarding

Key Algorithms

DUAL (Diffusing Update Algorithm): Loop-free path calculation
Feasibility Condition: Determines loop-free paths
Distance Calculation: Composite metric computation

Packet Types

Hello Packets

Function: Discover and maintain neighbors
Interval: 5 seconds (broadcast), 60 seconds (non-broadcast)
Destination: Multicast 224.0.0.10
Purpose: Maintain adjacency

Update Packets

Function: Send routing information
Trigger: Topology changes
Reliability: Guaranteed delivery
Content: Route advertisements

Query Packets

Function: Seek alternate paths
Trigger: Feasible successor unavailable
Reliability: Guaranteed delivery
Scope: May flood network

Reply Packets

Function: Respond to queries
Reliability: Guaranteed delivery
Trigger: Receipt of query
Destination: Originating router

ACK Packets

Function: Acknowledge reliable packets
Reliability: Unreliable delivery
Content: Empty packet
Purpose: Confirm receipt

EIGRP Metrics

Composite Metric Formula

TEXT

Metric = [K1*BW + (K2*BW)/(256-load) + K3*delay] * [K5/(reliability + K4)]

Default K Values

K1 = 1: Bandwidth weighting
K3 = 1: Delay weighting
K2 = K4 = K5 = 0: Disabled components

Simplified Formula

TEXT

Metric = Bandwidth + Delay

Metric Components

Bandwidth

Calculation: Minimum bandwidth along path
Formula: (10^7 / min_bandwidth) * 256
Unit: Kilobits per second
Impact: Lower bandwidth increases metric

Delay

Calculation: Cumulative delay along path
Formula: (sum_delay / 10) * 256
Unit: Tens of microseconds
Impact: Higher delay increases metric

Reliability

Range: 0-255 (percentage * 2.55)
Default: 255 (100% reliable)
Usage: Rarely used in metric
Impact: Lower reliability increases metric

Load

Range: 0-255 (percentage * 2.55)
Default: 1 (almost no load)
Usage: Rarely used in metric
Impact: Higher load increases metric

EIGRP Concepts

Successor and Feasible Successor

Successor

Definition: Best route to destination
Installation: Installed in routing table
Function: Primary forwarding path
Selection: Lowest metric route

Feasible Successor

Definition: Backup route meeting feasibility condition
Installation: Not in routing table (backup)
Function: Immediate replacement for successor
Condition: Advertised distance < feasible distance

Feasibility Condition

TEXT

Advertised Distance < Feasible Distance

Where Feasible Distance = Metric of current successor

Variance and Load Balancing

Equal Cost Load Balancing

Default: Up to 4 paths (configurable up to 16)
Requirement: Identical metrics
Function: Distribute traffic equally

Unequal Cost Load Balancing

Configuration: variance command
Requirement: Feasible successor exists
Function: Distribute traffic proportionally
Formula: Metric less than or equal to (Successor Metric × Variance)

EIGRP Configuration

Basic Configuration

TEXT

router eigrp 100
network 192.168.1.0
network 192.168.2.0
eigrp router-id 1.1.1.1

Advanced Configuration

Passive Interface

TEXT

passive-interface default
no passive-interface GigabitEthernet0/0

Authentication

TEXT

interface GigabitEthernet0/0
ip authentication mode eigrp 100 md5
ip authentication key-chain eigrp 100 KEYCHAIN_NAME

Summarization

TEXT

interface Serial0/0/0
ip summary-address eigrp 100 10.0.0.0 255.0.0.0

Stub Routing

TEXT

router eigrp 100
eigrp stub

Metric Manipulation

Changing Bandwidth

TEXT

interface GigabitEthernet0/0
bandwidth 100000

Changing Delay

TEXT

interface GigabitEthernet0/0
delay 10

Offset Lists

TEXT

router eigrp 100
offset-list 1 out 10 Serial0/0/0

EIGRP Timers

Key Timers

Hello Interval

Broadcast/Point-to-point: 5 seconds
Non-broadcast: 60 seconds
Adjustable: Configurable per interface
Purpose: Maintain neighbor adjacency

Hold Time

Default: 3 × Hello interval
Broadcast/Point-to-point: 15 seconds
Non-broadcast: 180 seconds
Purpose: Detect neighbor failure

Active Timer

Default: 3 minutes
Adjustable: Configurable
Purpose: Time to wait for replies to queries
Function: Prevent stuck-in-active condition

Timer Configuration

TEXT

interface GigabitEthernet0/0
ip hello-interval eigrp 100 10
ip hold-time eigrp 100 30

EIGRP Features

Route Summarization

Automatic Summarization

Default: Enabled (disabled in EIGRP Named Mode)
Function: Summarize at major network boundaries
Behavior: May cause suboptimal routing
Control: Manual summarization preferred

Manual Summarization

Granularity: Configurable per interface
Function: Control route advertisement
Benefits: Reduce routing table size
Location: Any interface in routing domain

Stub Routing

Features

Function: Limit query propagation
Types: Connected, static, summary, redistributed
Benefits: Faster convergence, reduced resource usage
Use Cases: Hub-and-spoke topologies

Configuration

TEXT

router eigrp 100
eigrp stub connected static summary redistributed

Authentication

Types

Simple Authentication: Plain text password
MD5 Authentication: Hash-based authentication
SHA Authentication: SHA-based authentication
Key Chains: Multiple keys with timing

Load Balancing

Equal Cost

Default: 4 paths maximum
Configurable: Up to 16 paths
Requirement: Identical metrics
Function: Distribute traffic evenly

Unequal Cost

Configuration: variance command
Requirement: Feasible successor
Function: Proportional traffic distribution
Formula: Metric less than or equal to (Successor × Variance)

EIGRP Security Considerations

Authentication Methods

Simple Authentication

Mechanism: Plain text password
Security: Weak, easily compromised
Usage: Minimal security requirement
Configuration: Simple to implement

MD5 Authentication

Mechanism: Hash-based authentication
Security: Stronger than simple authentication
Usage: Common security practice
Configuration: Requires shared key

SHA Authentication

Mechanism: SHA-based authentication
Security: Stronger than MD5
Usage: Modern security standard
Compatibility: Requires newer software

Security Best Practices

Enable Authentication: Always authenticate EIGRP packets
Use Strong Keys: Complex, regularly rotated passwords
Monitor Adjacencies: Track neighbor relationships
Filter Routes: Control route advertisement when appropriate

Troubleshooting EIGRP

Common Issues

Neighbor Problems

AS Number Mismatch: Must match between neighbors
K-Value Mismatch: Must match between neighbors
Authentication Failure: Auth must match
Network Statement Mismatch: Networks must be advertised
Hello/Hold Time Mismatch: Timers must match
MTU Mismatch: Interface MTUs must match

Convergence Problems

Stuck-in-Active: Queries not replied to
Route Flapping: Unstable routes
Suboptimal Paths: Not using best routes
Missing Routes: Routes not learned

Diagnostic Commands

Verification Commands

show ip eigrp neighbors: Display EIGRP neighbors
show ip eigrp topology: Show EIGRP topology table
show ip route eigrp: Show EIGRP routes
show ip eigrp interfaces: Show EIGRP interface status
show ip protocols: Show routing protocol configuration

Debug Commands

debug eigrp packets: Monitor EIGRP packets
debug eigrp fsm: Monitor finite state machine
debug eigrp events: Monitor EIGRP events
debug eigrp traffic: Monitor EIGRP traffic

Troubleshooting Process

Verify Physical Connectivity: Check interface status
Check EIGRP Process: Ensure EIGRP is running
Validate Configuration: Verify EIGRP settings
Examine Neighbor Status: Check adjacency formation
Analyze Topology Table: Verify route learning
Review Routing Table: Confirm route installation

Advanced EIGRP Features

Named Mode (EIGRP Named Configuration)

Function: Newer configuration method
Benefits: Better organization, VRF support
Compatibility: Runs alongside classic EIGRP
Features: Multiple autonomous systems per interface

Named Mode Configuration

TEXT

router eigrp NAME
!
address-family ipv4 unicast autonomous-system 100
!
topology base
metric maximum-hops 200
exit-af-topology
network 192.168.1.0 0.0.0.255

Wide Metrics

Function: Support for higher bandwidth links
Benefit: Accurate metrics for 10G+ links
Requirement: IOS 15.1(2)S or later
Activation: Automatically enabled in named mode

EIGRP for IPv6

Function: EIGRP for IPv6 networks
Differences: IPv6 addressing, different operation
Compatibility: Separate process from IPv4 EIGRP
Standards: RFC 8780 compliant

VRF Support

Function: EIGRP in VRF instances
Benefits: Multiple routing tables
Configuration: Named mode required
Isolation: Separate routing domains

EIGRP Scalability

Hierarchical Design

Summarization: Reduce routing information
Stub Routing: Limit query propagation
Network Design: Proper network statements
Address Planning: Efficient address allocation

Performance Considerations

Convergence Time: Fast convergence (sub-second)
Memory Usage: Topology table requires memory
CPU Usage: Moderate CPU requirements
Bandwidth Usage: Low bandwidth consumption

EIGRP vs Other Protocols

EIGRP vs OSPF

EIGRP: Cisco proprietary, hybrid protocol
OSPF: Open standard, link-state protocol
Convergence: Both converge quickly
Configuration: EIGRP simpler to configure

EIGRP vs RIP

EIGRP: Advanced distance-vector
RIP: Basic distance-vector
Convergence: EIGRP much faster
Features: EIGRP more advanced

EIGRP vs BGP

EIGRP: Interior gateway protocol
BGP: Exterior gateway protocol
Scale: EIGRP for AS, BGP for inter-AS
Complexity: BGP more complex

Best Practices

Network Design

AS Number Planning: Use consistent AS numbers
Network Statements: Plan network advertisements
Summarization: Implement route summarization
Metric Tuning: Adjust metrics appropriately

Configuration

Authentication: Always enable authentication
Timer Tuning: Adjust timers appropriately
Passive Interfaces: Configure as needed
Monitoring: Enable appropriate logging

Operational

Documentation: Maintain network diagrams
Monitoring: Track EIGRP metrics
Backup: Regular configuration backups
Testing: Regular failover testing

Future of EIGRP

Current Status

Open Standard: RFC 7868 (2016)
Continued Support: Cisco continues development
IPv6 Support: EIGRP for IPv6 available
Modern Features: Wide metrics, named mode

Evolution Considerations

SDN Integration: Programmable EIGRP
Enhanced Security: Stronger authentication
Performance: Further optimization
Cloud Integration: Cloud-aware routing

Conclusion

EIGRP is a sophisticated routing protocol that combines the simplicity of distance-vector protocols with the loop-prevention mechanisms of link-state protocols. Its fast convergence, efficient bandwidth usage, and advanced features make it suitable for complex enterprise networks. Understanding EIGRP operations, configuration, and troubleshooting techniques is essential for network administrators working in Cisco environments. As EIGRP transitions from proprietary to open standard, it continues to evolve while maintaining its position as one of the most popular interior gateway protocols.