Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol developed by Cisco. It is widely used in enterprise networks due to its fast convergence, scalability, and efficient use of bandwidth. EIGRP operates using Diffusing Update Algorithm (DUAL) to calculate the shortest path dynamically while ensuring loop-free routing.

In this guide, we will walk through the essential steps to configure EIGRP on Cisco devices, covering basic setup, verification, and optimization techniques.

1. Enabling EIGRP on a Router

1.1 Enable EIGRP and Assign an Autonomous System (AS) Number

EIGRP uses an AS number to group routers that should exchange routing information. To enable EIGRP, enter the global configuration mode:

Router(config)# router eigrp <AS_Number>

For example, if the AS number is 100:

Router(config)# router eigrp 100

1.2 Advertise Networks

Specify which networks should participate in EIGRP by using the network command.

Router(config-router)# network <network_address> <wildcard_mask>

For instance, if your network is 192.168.1.0/24:

Router(config-router)# network 192.168.1.0 0.0.0.255

1.3 Disable Automatic Summarisation (Optional)

By default, EIGRP summarises routes at major network boundaries. You can disable this with:

Router(config-router)# no auto-summary

2. Verifying EIGRP Configuration

2.1 Checking Neighbour Relationships

To confirm that routers are forming EIGRP neighbor relationships, use:

Router# show ip eigrp neighbors

This displays details about adjacent routers, including their IP addresses and uptime.

2.2 Viewing the EIGRP Routing Table

To check the routes learned via EIGRP:

Router# show ip route eigrp

This will display all routes discovered and installed in the routing table via EIGRP.

2.3 Checking Advertised Routes

To see which routes are being advertised to neighbors:

Router# show ip eigrp topology

This shows feasible successors (backup routes) and successor routes.

3. EIGRP Optimisation Techniques

3.1 Using Bandwidth and Delay for Route Calculation

EIGRP uses bandwidth and delay as primary metrics. You can modify the default interface settings to influence path selection:

Router(config-if)# bandwidth <kbps>

Router(config-if)# delay <tens_of_microseconds>

For example, to set bandwidth to 100 Mbps and delay to 10 microseconds:

Router(config-if)# bandwidth 100000

Router(config-if)# delay 10

3.2 Configuring EIGRP Authentication

To enhance security, configure EIGRP authentication to prevent unauthorized routers from forming neighbor relationships.

Step 1: Create a Keychain

Router(config)# key chain EIGRP-KEY

Router(config-keychain)# key 1

Router(config-keychain-key)# key-string mysecurekey

Step 2: Apply Authentication to an Interface

Router(config-if)# ip authentication mode eigrp 100 md5

Router(config-if)# ip authentication key-chain eigrp 100 EIGRP-KEY

3.3 Adjusting EIGRP Timers

EIGRP uses hello and hold timers to maintain neighbor relationships. You can fine-tune them for better network performance:

Router(config-if)# ip hello-interval eigrp 100 5

Router(config-if)# ip hold-time eigrp 100 15

This sets the hello timer to 5 seconds and the hold timer to 15 seconds.

4. Troubleshooting EIGRP Issues

4.1 Checking EIGRP Interface Status

Ensure that EIGRP is enabled on the correct interfaces:

Router# show ip eigrp interfaces

4.2 Verifying EIGRP Packet Exchange

To check if EIGRP hello and update packets are being sent and received:

Router# debug eigrp packets

4.3 Checking for Stuck-in-Active (SIA) State

EIGRP routes may become stuck in active (SIA) if queries are not being answered. Use:

Router# show ip eigrp topology active

If an SIA condition occurs, consider adjusting the active timer:

Router(config-router)# timers active-time 180

This extends the active query timeout to 180 seconds.

Conclusion

Configuring EIGRP is a critical skill for network engineers working with Cisco devices. By understanding its fundamental commands, verification techniques, and optimisation strategies, you can ensure efficient and reliable routing within your network.

This guide covered essential EIGRP setup, troubleshooting, and security best practices to help you build a scalable and high-performance network.