Basic EIGRP Configuration

Getting Started with EIGRP

EIGRP configuration is straightforward and requires minimal commands. This section covers the essential configuration steps to get EIGRP running on your network.

Basic EIGRP Configuration Steps

Configuration Process

Step 1

Enable EIGRP
with AS number

Step 2

Configure
network statements

Step 3

Verify
configuration

Step 4

Test
connectivity

Minimal EIGRP Configuration

Basic Configuration Template

# Enter EIGRP configuration mode
router eigrp [autonomous-system-number]

# Advertise networks
network [network-address] [wildcard-mask]

# Example:
router eigrp 100
 network 192.168.1.0 0.0.0.255
 network 10.0.0.0 0.255.255.255
 network 172.16.0.0 0.0.255.255

Autonomous System Number

Aspect Description Range Best Practice
AS Number Identifies EIGRP routing domain 1-65535 Use consistent number across domain
Scope Local significance only Per routing domain Document AS number usage
Neighbor Formation Must match for adjacency Exact match required Verify AS number consistency

Network Statement Configuration

Network Statement Syntax

# Basic network statement
network [network-address]

# Network statement with wildcard mask
network [network-address] [wildcard-mask]

# Examples:
network 192.168.1.0          # Assumes classful mask
network 192.168.1.0 0.0.0.255  # Explicit wildcard mask
network 0.0.0.0 255.255.255.255 # Enable on all interfaces

Network Statement Effects

Interface Enablement

Enables EIGRP on matching interfaces

Route Advertisement

Advertises connected networks

Neighbor Discovery

Sends Hello packets on interfaces

Adjacency Formation

Forms neighbor relationships

Complete Configuration Example

Network Topology

    Router A (192.168.1.1/24)
         |
         | Gi0/0
         |
    Router B (192.168.1.2/24)
         |
         | Gi0/1 (10.1.1.1/30)
         |
    Router C (10.1.1.2/30)
         |
         | Gi0/2 (172.16.1.1/24)
         |
    LAN (172.16.1.0/24)

Router A Configuration

Router A EIGRP Config

hostname RouterA
!
interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 no shutdown
!
router eigrp 100
 network 192.168.1.0 0.0.0.255
 no auto-summary

Router B Configuration

Router B EIGRP Config

hostname RouterB
!
interface GigabitEthernet0/0
 ip address 192.168.1.2 255.255.255.0
 no shutdown
!
interface GigabitEthernet0/1
 ip address 10.1.1.1 255.255.255.252
 no shutdown
!
router eigrp 100
 network 192.168.1.0 0.0.0.255
 network 10.1.1.0 0.0.0.3
 no auto-summary

Router C Configuration

Router C EIGRP Config

hostname RouterC
!
interface GigabitEthernet0/1
 ip address 10.1.1.2 255.255.255.252
 no shutdown
!
interface GigabitEthernet0/2
 ip address 172.16.1.1 255.255.255.0
 no shutdown
!
router eigrp 100
 network 10.1.1.0 0.0.0.3
 network 172.16.1.0 0.0.0.255
 no auto-summary

Configuration Best Practices

EIGRP Configuration Guidelines

  • Plan AS Numbers: Use consistent AS numbers across the routing domain
  • Use Passive Interfaces: Secure interfaces that don't need EIGRP neighbors
  • Configure Router ID: Set explicit Router ID for stability
  • Document Configuration: Maintain clear documentation of network statements
  • Test Thoroughly: Verify configuration with connectivity tests

Wildcard Masks

Understanding Wildcard Masks

Wildcard masks in EIGRP network statements determine which interfaces participate in the EIGRP process. They are the inverse of subnet masks and use different logic.

Wildcard Mask Fundamentals

Aspect Subnet Mask Wildcard Mask Function
Binary Logic 1 = network, 0 = host 0 = match, 1 = ignore Opposite logic
Purpose Define network boundaries Define matching criteria Interface selection
Calculation Network design 255.255.255.255 - subnet mask Inverse relationship

Wildcard Mask Calculation

Calculation Formula

Wildcard Mask = 255.255.255.255 - Subnet Mask

Examples:

  • Subnet mask 255.255.255.0 → Wildcard mask 0.0.0.255
  • Subnet mask 255.255.255.252 → Wildcard mask 0.0.0.3
  • Subnet mask 255.255.0.0 → Wildcard mask 0.0.255.255

Common Wildcard Mask Examples

Subnet Mask CIDR Wildcard Mask Use Case
255.255.255.255 /32 0.0.0.0 Exact host match
255.255.255.252 /30 0.0.0.3 Point-to-point links
255.255.255.0 /24 0.0.0.255 Standard Class C networks
255.255.0.0 /16 0.0.255.255 Class B networks
255.0.0.0 /8 0.255.255.255 Class A networks
0.0.0.0 /0 255.255.255.255 Match all interfaces

Auto Summary

Understanding Auto Summary

Auto Summary is a legacy EIGRP feature that automatically summarizes routes to their classful boundaries. While disabled by default in modern IOS versions, understanding its behavior is crucial for network troubleshooting.

What is Auto Summary?

Auto Summary automatically creates summary routes at classful network boundaries (Class A, B, and C). When enabled, EIGRP advertises only the major network instead of individual subnets when crossing classful boundaries.

Important Note

Auto Summary is disabled by default in modern Cisco IOS versions. It should generally remain disabled to avoid routing issues in modern networks with VLSM and CIDR.

Configuration Verification

Verifying EIGRP Operation

Proper verification ensures EIGRP is working correctly and helps identify potential issues before they impact network performance. This section covers essential verification commands and techniques.

Basic Verification Commands

Essential EIGRP Verification

# Check EIGRP configuration
show running-config | section eigrp

# View EIGRP processes and settings
show ip protocols

# Check enabled interfaces
show ip eigrp interfaces

# View neighbor table
show ip eigrp neighbors

# Check topology table
show ip eigrp topology

# View routing table
show ip route eigrp

Common Configuration Mistakes

Configuration Pitfalls

  • Mismatched AS Numbers: Neighbors with different AS numbers won't form adjacencies
  • Incorrect Wildcard Masks: May enable EIGRP on wrong interfaces
  • Auto-Summary Issues: Can cause routing problems in discontiguous networks
  • Passive Interface Confusion: Accidentally making required interfaces passive
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