Common EIGRP Issues

EIGRP Troubleshooting Overview

Understanding common EIGRP issues and their solutions helps maintain network stability and performance. This section covers the most frequent problems and their diagnostic approaches.

Categories of EIGRP Issues

Neighbor Problems

Adjacency formation and maintenance issues

Convergence Issues

Slow convergence and SIA conditions

Route Problems

Missing routes and suboptimal paths

Performance Issues

High CPU usage and memory consumption

Top 10 EIGRP Issues

#	Issue	Symptoms	Common Cause	Quick Fix
1	No neighbors	Empty neighbor table	Missing network statement	Add network statement
2	Neighbors flapping	Neighbors going up/down	Timer mismatch or link issues	Check timers and physical layer
3	Stuck in Active (SIA)	Routes stuck in active state	Query propagation issues	Use stub configuration
4	Missing routes	Expected routes not in table	Filtering or metric issues	Check distribute lists
5	Suboptimal routing	Traffic taking poor paths	Incorrect metrics	Adjust bandwidth/delay
6	Auto-summary problems	Connectivity issues	Classful summarization	Disable auto-summary
7	K-value mismatch	Neighbors won't form	Different metric weights	Standardize K-values
8	Authentication failure	Adjacency fails	Key mismatch or expiration	Verify key configuration
9	High CPU usage	Router performance issues	Excessive route churn	Implement summarization
10	Memory exhaustion	Router crashes or slowness	Too many routes	Filter routes or add memory

Systematic Troubleshooting Approach

Troubleshooting Methodology

Step 1

Identify the
problem symptoms

Step 2

Gather information
using show commands

Step 3

Analyze configuration
and logs

Step 4

Implement solution
and verify

Essential Troubleshooting Commands

First Response Commands

# Check EIGRP neighbors
show ip eigrp neighbors

# View routing table
show ip route eigrp

# Check EIGRP topology
show ip eigrp topology

# Verify EIGRP configuration
show ip protocols

# Check interface status
show ip eigrp interfaces

# View recent events
show ip eigrp events

Neighbor Problems

EIGRP Neighbor Issues

Neighbor relationship problems are among the most common EIGRP issues. Understanding adjacency requirements and troubleshooting techniques is essential for maintaining stable EIGRP operations.

EIGRP Neighbor Requirements

Requirement	Description	Verification Command	Common Issue
Same AS Number	Identical autonomous system number	show ip protocols	Mismatched AS configuration
Same Subnet	Interfaces on same network	show ip interface brief	IP addressing errors
Authentication	Matching authentication keys	show ip eigrp neighbors detail	Key mismatch or expiration
K-values	Identical metric weights	show ip protocols	Different K-value configuration
Primary Subnet	No secondary IP addresses	show ip interface	Secondary IP on interface

Common Neighbor Problems

Problem 1: No Neighbors Discovered

Diagnostic Commands

# Check neighbor table
show ip eigrp neighbors

# Verify interfaces are enabled for EIGRP
show ip eigrp interfaces

# Check network statements
show ip protocols

# Verify interface IP addresses
show ip interface brief

# Check interface status
show interfaces | include "line protocol"

Problem 2: Neighbors Flapping

Flapping Diagnosis

# Check neighbor uptime
show ip eigrp neighbors

# View neighbor events
show ip eigrp events | include neighbor

# Check interface errors
show interfaces | include error

# Monitor neighbor changes
debug ip eigrp neighbor

# Check hold timer values
show ip eigrp neighbors detail

Convergence Issues

EIGRP Convergence Problems

Convergence issues can significantly impact network performance and user experience. Understanding the causes and solutions for slow convergence and SIA conditions is crucial for network stability.

Types of Convergence Issues

Slow Convergence

Routes take too long to converge after topology changes

Stuck in Active (SIA)

Routes remain in active state beyond timer threshold

Route Flapping

Routes continuously change between active and passive

Partial Convergence

Some routes converge while others remain unstable

Stuck in Active (SIA) Condition

SIA Overview

SIA occurs when a router goes active for a route but doesn't receive all expected reply messages within the active timer (default 3 minutes). This triggers neighbor relationship resets and can cause network instability.

SIA Troubleshooting

SIA Detection Commands

# Check for active routes
show ip eigrp topology active

# View SIA events
show ip eigrp events | include SIA

# Check active timer setting
show ip protocols | include "active"

# Monitor specific route
show ip eigrp topology 192.168.1.0

# Sample SIA output:
# A 192.168.1.0/24, 0 successors, FD is Inaccessible
#     via 10.1.1.2 (Infinity/Infinity), Serial0/0/0
#     Remaining replies:
#       via 10.1.1.2, Serial0/0/0

SIA Prevention Strategies

Route Summarization for SIA Prevention

# Summarize at distribution layer
interface serial0/0/0
 ip summary-address eigrp 100 192.168.0.0 255.255.248.0

# This summary:
# - Reduces routing table size
# - Limits query propagation
# - Improves convergence time
# - Reduces SIA risk

# Verify summarization effect
show ip eigrp topology summary

Stub Configuration for SIA Prevention

Stub Router Configuration

# Configure spoke routers as stub
router eigrp 100
 eigrp stub connected summary

# Benefits:
# - Hub doesn't send queries to stub routers
# - Reduces query scope
# - Faster convergence
# - Lower SIA risk

# Verify stub configuration
show ip eigrp neighbors detail | include Stub

Debug Commands

EIGRP Debug Commands

Debug commands provide real-time insight into EIGRP operations but should be used carefully as they can impact router performance. This section covers the most useful debug commands and their applications.

Debug Command Categories

Neighbor Debug

Monitor neighbor relationships and hello packets

Packet Debug

Analyze EIGRP packet processing

FSM Debug

Track DUAL finite state machine operations

Event Debug

Monitor routing events and changes

Essential Debug Commands

Common Debug Commands

# Debug neighbor formation and maintenance
debug ip eigrp neighbor

# Debug hello packets
debug eigrp packets hello

# Debug DUAL FSM operations
debug eigrp fsm

# Debug all EIGRP packets (use carefully)
debug eigrp packets

# Debug specific route
debug ip eigrp 192.168.1.0 255.255.255.0

# Turn off all debugging
undebug all
no debug all

Debug Safety and Best Practices

Debug Safety Warning

Performance Impact: Debug commands can significantly impact router performance
Production Use: Use sparingly in production environments
Logging Buffer: Ensure adequate logging buffer size
Time Limits: Enable debugging for short periods only
Cleanup: Always disable debugging when finished

Debug Preparation

Prepare for Debug Session

# Increase logging buffer size
logging buffered 32768

# Set timestamp format
service timestamps debug datetime msec

# Disable console logging during debug
no logging console

# Enable terminal monitoring
terminal monitor

# Check current debug status
show debugging

Debug Command Reference

Command	Purpose	Performance Impact	Use Case
debug ip eigrp neighbor	Monitor neighbor events	Low	Neighbor relationship issues
debug eigrp packets hello	Track hello packets	Medium	Neighbor discovery problems
debug eigrp fsm	DUAL state machine	High	Route computation issues
debug eigrp packets	All packet types	Very High	Comprehensive troubleshooting
debug ip eigrp [network]	Specific route debugging	Medium	Route-specific issues

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