Common Reasons for Failed Neighbor Adjacency in EIGRP

In EIGRP (Enhanced Interior Gateway Routing Protocol), establishing neighbor adjacencies is essential for proper routing within a network. However, various factors can lead to the failure of neighbor adjacencies. Understanding these reasons is crucial for effective network troubleshooting. Here are some common causes for failed neighbor adjacencies:

1. Mismatched Autonomous System (AS) Numbers:

• Routers in the same EIGRP network must share the same AS number. A mismatch can prevent adjacency formation.

2. Mismatched K-Values:

• Inconsistent K-values in EIGRP configurations between neighboring routers can lead to adjacency issues.

3. VLAN Access Control List (VACL):

• If a VLAN Access Control List (VACL) is improperly configured, it can block EIGRP traffic and disrupt adjacency formation.

4. Access Control List (ACL):

• Misconfigured Access Control Lists (ACLs) may inadvertently filter EIGRP traffic, causing adjacency problems.

5. Incorrect 'Network' Statements:

• Incorrectly configured 'network' statements in EIGRP configurations can prevent routers from recognizing each other as neighbors.

6. Passive Interfaces:

• Interfaces configured as 'passive' do not participate in EIGRP neighbor discovery. Verify that the intended interfaces are not passive.

7. Authentication:

• Authentication misconfiguration or mismatched authentication keys can hinder neighbor adjacencies from forming.

8. Incorrect Subnets:

• Ensuring that the interfaces are in the correct subnets is crucial for neighbor discovery. Misconfigured subnets can disrupt the adjacency process.

9. Interface is Down:

• If an interface participating in EIGRP is down or not properly connected, it will not be able to establish a neighbor adjacency.

10. Timers:

• Timers in EIGRP, such as Hello and Hold timers, must match between routers for successful adjacency formation.

Verification Commands:

To diagnose and troubleshoot neighbor adjacency issues in EIGRP, utilize the following verification commands:

• show ip eigrp neighbor [detail]: This command displays detailed information about EIGRP neighbors, including their status and addresses.

• show cdp neighbors [detail]: The Cisco Discovery Protocol (CDP) can provide insights into neighboring devices, helping identify potential adjacency problems.

• show ip interface brief: This command lists the status of all interfaces, allowing you to check for down interfaces.

• show ip eigrp traffic: It provides information about EIGRP traffic statistics, helping you identify potential issues.

• show ip interface [int] | inc access: Check interfaces for access control lists (ACLs) that may be affecting EIGRP traffic.

• show access-lists: Examine the configured access control lists (ACLs) to ensure they are not blocking EIGRP packets.

• show ip protocols: This command offers insights into the routing protocols' configuration, including EIGRP.

• show run int [int]: View the running configuration of specific interfaces to check for misconfigurations.

• show ip eigrp interface [detail]: Check the configuration of EIGRP on individual interfaces to identify potential issues.

• show key chain: Inspect the key chain configuration if authentication is in use to ensure correctness.

• show run | sec router eigrp: View the EIGRP configuration section to identify potential misconfigurations.

• debug eigrp packets [terse] [detail] and debug eigrp packets hello detail: Enabling these debug commands provides real-time packet-level information for detailed troubleshooting.

In conclusion, understanding the common reasons for failed EIGRP neighbor adjacencies and using the appropriate verification commands is essential for identifying and resolving network issues, ensuring stable and efficient routing in your EIGRP network.