EIGRP Deep Dive: Configuration, Metrics, and Path Selection

Introduction to EIGRP

EIGRP (Enhanced Interior Gateway Routing Protocol) is a hybrid routing protocol that combines features of both link-state and distance-vector protocols. Originally designed for personal use, its popularity has made it an open-source protocol. Unlike traditional routing protocols, EIGRP does not rely on periodic updates; instead, it only sends updates when there are changes in the network topology.

EIGRP Tables

EIGRP maintains three primary tables to manage routing information:

Path Selection in EIGRP

EIGRP selects the best path based on a composite metric value, calculated using the following formula:

Components of the Metric:

Bandwidth: Derived from the slowest link in the path (in Kbps)

Bandwidth = 10^7 / Bandwidth_minimum

Delay: Sum of all delays along the path (in microseconds)

Delay = (Sum of Delays) / 10

Administrative Distance in EIGRP

Administrative Distance (AD) is a measure of a route's trustworthiness. In EIGRP:

Configuring EIGRP

EIGRP can be configured using two methods:

1. Classical Mode (Legacy)

router eigrp [AS_number]

router eigrp 100

Network Addition:

For classful networks:

network [network_ID]

Example:

network 192.168.10.0

For classless networks (using wildcard mask):

network [network_ID] [wildcard_mask]

Example:

network 200.1.12.0 0.0.0.3

2. Named Mode (Modern)

router eigrp [name]

router eigrp mysigrp

Practical Configuration Example

Network Topology

EIGRP topology with two routers and connected networks

R1

Fa0/0: 192.168.10.1/24

S5/0: 200.1.12.1/30

Serial Link

R2

S5/0: 200.1.12.2/30

Fa0/0: 192.168.20.1/24

R1 Configuration:

R1(config)# interface FastEthernet0/0
R1(config-if)# ip address 192.168.10.1 255.255.255.0
R1(config-if)# no shutdown
R1(config-if)# exit

R1(config)# interface Serial5/0
R1(config-if)# ip address 200.1.12.1 255.255.255.252
R1(config-if)# no shutdown
R1(config-if)# exit

R1(config)# router eigrp 100
R1(config-router)# network 192.168.10.0
R1(config-router)# network 200.1.12.0 0.0.0.3
R1(config-router)# exit

R2 Configuration:

R2(config)# interface FastEthernet0/0
R2(config-if)# ip address 192.168.20.1 255.255.255.0
R2(config-if)# no shutdown
R2(config-if)# exit

R2(config)# interface Serial5/0
R2(config-if)# ip address 200.1.12.2 255.255.255.252
R2(config-if)# no shutdown
R2(config-if)# exit

R2(config)# router eigrp 100
R2(config-router)# network 192.168.20.0
R2(config-router)# network 200.1.12.0 0.0.0.3
R2(config-router)# exit

Routing Tables:

R1's routing table showing EIGRP-learned routes

R2's routing table showing EIGRP-learned routes

Connectivity Verification:

Successful ping from PC1 (192.168.10.5) to PC2 (192.168.20.5)

Successful ping from PC2 (192.168.20.5) to PC1 (192.168.10.5)

Commands: To View Different EIGRP Tables

# View Routing Table
show ip route

# View EIGRP Routes
show ip route eigrp

# View Topology Table
show ip eigrp topology

# View Neighbor Table
show ip eigrp neighbors

Path Manipulation in EIGRP

EIGRP allows administrators to influence path selection by adjusting Bandwidth and Delay on egress interfaces (interfaces forwarding traffic out of the router).

Configuration Example:

R1(config)# interface Serial5/0
R1(config-if)# bandwidth 1444
R1(config-if)# delay 50
R1(config-if)# exit

Interface Configuration Verification:

We can see BW 1444 Kbit/sec, DLY 500 usec are now changed by command

Note: The router multiplies the delay value by 10. For example, delay 50 equals 500 microseconds.

Conclusion

EIGRP is a powerful and flexible routing protocol suitable for modern networks. Its hybrid nature, efficient update mechanism, and advanced features like path manipulation make it a preferred choice for many network administrators.

By understanding its tables, metrics, and configuration methods, you can optimize EIGRP for your specific network requirements.