eigrp scenario based interview questions and answers

Scenario 1 - EIGRP Neighbor Not Coming Up

Possible Causes:

1. AS number mismatch between neighbors.

2. K value mismatch between routers.

3. IP is on different subnets.

4. Primary and secondary address on the interface mismatch between routers.

5. Layer 2 switch/hub connecting different subnet devices in the same VLAN (e.g., VLAN leaking from VLAN 1 to VLAN 2 by using a back-to-back cable).

6. Access-list blocking EIGRP multicast packets on the interface.

CLI Command:show ip eigrp neighbors

Solution:

1. Ensure the same AS number is in use on both routers.

2. Ensure the same K value is configured on both routers.

3. Ensure both routers are in the same IP subnet.

4. Ensure that primary addresses match on both sides and secondary addresses match on secondary sides.

5. Ensure there is no VLAN leaking, and the devices are connected correctly.

Scenario 2 - EIGRP Neighbor is Established but Not Exchanging Routes and Getting Reset

Possible Causes:

1. Mismatch in subnet mask between the neighbors.

2. Hold time expired.

3. Retry limit exceeded.

4. VLAN leak happening or cross-VLAN domain due to some redundant link or wrong cabling.

CLI Command:show ip eigrp neighbors

Solution:

1. Ensure there is no mismatch in the subnet mask between neighbors.

2. Check for packet loss on ports (End-CPU to End-CPU).

3. Ensure unicast reachability is available and pinging works end-to-end.

4. Ensure no access-list is blocking the communication between neighbors.

5. Ensure COPP (Control Plane Policing) is not blocking the unicast packets between neighbors.

6. Ensure there is no MTU mismatch between the neighbors.

7. Ensure EIGRP neighbors are on the same subnet, and no VLANs are leaking to other subnets.

Note: Mismatch in subnet masks can cause routing loops in the network.

Scenario 3 - Router Stuck in Active

Possible Causes:

1. Bad or congested links.

2. Low router resources (e.g., low memory or high CPU).

3. Long query range.

4. Excessive redundancy.

5. Access-list or COPP blocking unicast packets between neighbors.

CLI Command:show ip eigrp topology activeshow ip eigrp neighbors

Solution:

1. Use summarization to reduce the scope of queries.

2. Consider using EIGRP stub to limit query propagation.

3. Implement a hierarchical network design to reduce the number of queries and improve scalability.

Scenario 4 - Routes Not Advertising to Neighbor

Possible Causes:

1. Access-list with distribute list is misconfigured.

2. Routers have a discontiguous network.

3. EIGRP split horizon is enabled.

CLI Command:debug ip eigrp

Solution:

1. Ensure the access-list allows the networks in the distribute list.

2. Make sure no auto-summary is enabled to prevent automatic summarization.

3. Disable EIGRP split horizon on the hub router in a hub-and-spoke topology.

4. Alternatively, use sub-interfaces with different subnets between hub and spoke routers.