Troubleshooting EIGRP

A Framework for Troubleshooting

A structured methodology is the key to efficient troubleshooting. Before diving into specific commands, it's helpful to have a mental map of the process. The first question is always: "Is the problem with the neighbor adjacency or with the routes themselves?"

This flowchart provides a high-level overview of the diagnostic steps we'll be following today.

Our lesson will be structured around the two main branches of this flowchart: neighbor failures and route propagation problems.

1. Troubleshooting Neighbor Adjacency Failures

When two routers fail to become EIGRP neighbors, the show ip eigrp neighbors command will show an empty table or a "flapping" neighbor that appears and disappears. This is always the first place to look.

The cause is typically a mismatch in a fundamental parameter required for adjacency or a connectivity issue. Here is a checklist of the most common causes:

• Layer 1/2/3 Connectivity: The interface is down, there's no physical link, or the IP addresses are in different subnets.
• Mismatched AS Numbers: Routers must be in the same EIGRP autonomous system.
• Mismatched K-Values: The metric calculation weights must be identical.
• Passive Interface: The interface connecting to the neighbor is mistakenly configured as passive, which suppresses Hello packets.
• Authentication Mismatch: The key-chain, key ID, or key string do not match.
• Access Control List (ACL): An ACL on an interface is blocking EIGRP multicast (224.0.0.10) or unicast packets.

Let's watch a video that demonstrates how to methodically diagnose several of these common adjacency problems.

Deeper Dive: When Hellos Work but Adjacency Fails

A more subtle issue occurs when Hellos (which are multicast) are exchanged, but the adjacency still fails to complete. This often manifests as a retry limit exceeded log message. This error points to a failure in EIGRP's reliable transport protocol, which uses unicast packets for updates, queries, and replies. If these unicast packets or their acknowledgements are lost, the adjacency will eventually be torn down.

A common culprit is an ACL that permits multicast traffic but inadvertently blocks the unicast EIGRP traffic between the neighbors.

2. Troubleshooting Route Propagation

Once the neighborship is established, the next step is to exchange routes. A route propagation problem occurs when show ip eigrp neighbors looks correct, but a specific network is missing from the routing table.

The key is to determine where the route is being lost. Is it never being advertised? Is it being filtered? Or is it being learned but not installed in the routing table?

To explore these scenarios, we will consult a definitive Cisco guide.

Let's summarize the common causes for missing routes:

• Filtering: A distribute-list is blocking the route from being advertised or learned.
• Summarization: The specific route is being aggregated into a summary. The summarizing router will have the specific routes, but its neighbors will only see the summary. (This links back to our previous lesson).
• Split Horizon: The router learns a route via an interface and will not advertise it back out the same interface. This is expected behavior but can cause issues in certain hub-and-spoke topologies.
• Stub Routing: A router is configured as a stub, which limits the routes it advertises to its neighbors (e.g., it may only advertise connected and summary routes). You can see this status in the output of show ip eigrp neighbors detail.
• Administrative Distance (AD): The router is learning the same prefix from a different source with a lower (better) AD, so the EIGRP route is not installed in the routing table. EIGRP internal routes have an AD of 90.

3. A Special Case: Stuck-in-Active (SIA)

A dreaded EIGRP condition is "Stuck-in-Active." This occurs when a router loses its successor for a route and sends a Query to its neighbors, but one or more neighbors fail to send a Reply within the active timer (about 3 minutes).

• Symptom: You will see a DUAL-3-SIA error message in the logs, and the router will tear down the adjacency with the non-responsive neighbor.
• Cause: SIA is a symptom of an underlying problem, not the problem itself. It's usually caused by:
  • Extremely congested or poor-quality links where replies are dropped.
  • Unidirectional links where queries are received but replies cannot be sent back.
  • An overly large query domain, where queries must traverse too many routers.

The best solution for SIA is prevention through good design. This is where our previous lesson on summarization is critical. By creating summarization boundaries, you limit the query domain and drastically reduce the chances of an SIA event occurring. Configuring branch routers as EIGRP stubs has the same preventative effect.

Conclusion

In this lesson, we developed a structured approach to diagnosing EIGRP problems. By systematically checking for adjacency requirements and then for route propagation issues, you can efficiently isolate and resolve most common EIGRP faults.

Key Takeaways:

• Troubleshooting is a two-step process: First, verify neighbor adjacencies (show ip eigrp neighbors). Second, verify route propagation (show ip route and show ip eigrp topology).
• Adjacency Failures are typically caused by misconfigurations like mismatched AS numbers, K-values, authentication, or subnet masks. ACLs blocking EIGRP packets are a more subtle cause.
• Route Propagation Issues occur when adjacencies are up but routes are missing. Common causes include filtering with distribute-list, split horizon, stub router configuration, and a route being learned via another protocol with a better Administrative Distance.
• The show ip protocols command is your best friend for checking many EIGRP parameters in one place, including AS, K-values, passive interfaces, and filtering.

Preview of the Next Lesson:

This concludes our deep dive into EIGRP. You now understand how it forms adjacencies, calculates metrics, selects paths, summarizes routes, and how to troubleshoot it. We will now move on to the other major interior gateway protocol in the CCNP curriculum: OSPF. In our next lesson, we will begin by analyzing the fundamental components of OSPF, including its packet types, neighbor state machine, and how it floods link-state information—drawing comparisons to the concepts we've just mastered in EIGRP.