Introduction

Welcome back to Network ThinkTank, where we cover everything related to networking. Today, we’ll explore Enhanced Interior Gateway Routing Protocol (EIGRP), a popular routing protocol used in many networks. When working with EIGRP, you might encounter issues related to neighbors, adjacencies, or routing. In this blog post, we’ll discuss common EIGRP problems, provide example configurations, and guide you through the troubleshooting process.

Issue 1: EIGRP Neighbors Not Forming Adjacency

EIGRP routers must establish adjacencies with their neighbors to exchange routing information. If EIGRP neighbors are not forming an adjacency, several factors could be causing the problem:

1. Mismatched Autonomous System (AS) numbers
2. Mismatched EIGRP K-values
3. Mismatched authentication settings

Example Configuration & Troubleshooting:

R1:

interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.0
!
router eigrp 100
network 10.0.0.0

R2:

interface FastEthernet0/0
ip address 10.0.0.2 255.255.255.0
!
router eigrp 200
network 10.0.0.0

In this example, R1 and R2 have mismatched AS numbers (100 on R1 and 200 on R2). To fix this issue, adjust the AS numbers to match:

R1:

router eigrp 100

R2:

router eigrp 100

Issue 2: EIGRP Routes Not Being Advertised

If EIGRP routes are not being advertised, the issue could be related to the following reasons:

1. Incorrect network statements in the EIGRP configuration
2. EIGRP not enabled on the correct interfaces
3. Passive interface configuration

Example Configuration & Troubleshooting:

R1:

interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.0
!
interface FastEthernet0/1
ip address 192.168.1.1 255.255.255.0
!
router eigrp 100
network 10.0.0.0
passive-interface FastEthernet0/1

R2:

interface FastEthernet0/0
ip address 10.0.0.2 255.255.255.0
!
router eigrp 100
network 10.0.0.0

In this example, R1 has a passive interface configured for FastEthernet0/1 (192.168.1.1/24), which prevents EIGRP from advertising the 192.168.1.0/24 network. To fix this issue, remove the passive interface configuration:

R1:

router eigrp 100
no passive-interface FastEthernet0/1

Alternatively, you can add a network statement to include the 192.168.1.0/24 network in EIGRP:

R1:

router eigrp 100
network 192.168.1.0

By addressing these issues, EIGRP should now function correctly, and routers should establish adjacencies and exchange routing information as expected.

Conclusion

Troubleshooting EIGRP can be a complex task, but with a systematic approach and a strong understanding of the protocol’s inner workings, you can efficiently identify and resolve issues. Remember to check for mismatched configurations, incorrect network statements, and passive interface settings when troubleshooting EIGRP problems. Keep these tips in mind, and you’ll be well on your way to mastering EIGRP troubleshooting.

Stay tuned to Network ThinkTank for more tips, tricks, and insights into the world of networking. Happy troubleshooting!