Networking

Top 50+ EIGRP Interview Questions and Answers

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🧠 What is EIGRP?

EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance-vector routing protocol developed by Cisco, widely used in enterprise networks. Known for its fast convergence, low bandwidth usage, and scalability, EIGRP is a common topic in networking interviews — especially for network engineers, system admins, L1–L3 support roles, and Cisco-certified professionals.

This post covers 50+ EIGRP interview questions, from basic to advanced levels, designed to help you crack roles requiring Cisco and dynamic routing knowledge.

📋 Table of Contents

  1. EIGRP Basics (L1)

  2. Intermediate-Level Questions (L2)

  3. Advanced EIGRP Concepts (L3/NOC)

  4. Real-World Troubleshooting Scenarios

  5. EIGRP vs OSPF & RIP

  6. Conclusion & Pro Tips

  7. FAQs

✅ Section 1: Basic EIGRP Interview Questions (L1 Level)

1. What is EIGRP?

Answer:
EIGRP is a Cisco-proprietary dynamic routing protocol that combines the advantages of distance-vector and link-state protocols. It uses DUAL (Diffusing Update Algorithm) for fast convergence.

2. What type of routing protocol is EIGRP?

Answer:
EIGRP is classified as an Advanced Distance Vector protocol, also known as a hybrid routing protocol.

3. Which protocol number is used by EIGRP?

Answer:
EIGRP uses Protocol number 88.

4. Which Layer of OSI Model does EIGRP operate on?

Answer:
EIGRP operates on Layer 3 (Network Layer).

5. What are the key features of EIGRP?

  • Uses DUAL algorithm

  • Supports VLSM and CIDR

  • Classless routing

  • Partial updates

  • Fast convergence

  • Unequal cost load balancing

6. Which multicast address does EIGRP use?

Answer:
EIGRP uses 224.0.0.10 as the multicast address to communicate with neighbors.

7. What is AS number in EIGRP?

Answer:
AS (Autonomous System) number is a logical identifier that groups routers running the same EIGRP instance.

8. What is the default administrative distance of EIGRP?

Answer:

  • 90 for internal routes

  • 170 for external routes

  • 5 for summary routes

9. Does EIGRP support VLSM?

Answer:
Yes, EIGRP supports VLSM (Variable Length Subnet Masking) and classless routing.

10. Which transport protocol does EIGRP use?

Answer:
EIGRP uses IP (Protocol 88) directly — it does not use TCP or UDP.

⚙️ Section 2: Intermediate EIGRP Interview Questions (L2 Level)

11. What is the role of the DUAL algorithm in EIGRP?

Answer:
DUAL (Diffusing Update Algorithm) ensures loop-free and fast convergence by identifying the best and backup paths to a destination.

12. Explain Successor and Feasible Successor.

  • Successor: The primary best path to the destination.

  • Feasible Successor: A backup path that meets the feasibility condition.

13. What is the Feasibility Condition?

Answer:
A route is a feasible successor if its advertised distance (AD) is less than the feasible distance (FD) of the current successor.

14. What are EIGRP packet types?

  • Hello – For neighbor discovery

  • Update – For routing info exchange

  • Query – When searching for a route

  • Reply – In response to query

  • ACK – Acknowledgement for reliable delivery

15. What is Split Horizon?

Answer:
Split Horizon is a technique that prevents routing loops by not advertising a route back to the interface it was learned from.

16. Can EIGRP perform unequal-cost load balancing?

Answer:
Yes. EIGRP is the only protocol that supports unequal-cost load balancing using the variance command.

17. What is the default Hello and Hold Timer in EIGRP?

  • Hello Timer: 5 seconds (on high-speed links)

  • Hold Timer: 15 seconds (by default)

18. How does EIGRP form neighbor relationships?

Answer:
EIGRP routers become neighbors if:

  • Hello packets are exchanged

  • AS number matches

  • Authentication (if used) matches

  • K-values match

19. What are K-values in EIGRP?

Answer:
K-values are constants used in EIGRP’s metric calculation:

  • K1 = Bandwidth

  • K2 = Load

  • K3 = Delay

  • K4, K5 = Reliability & MTU

Default: K1 = 1, K3 = 1; others = 0

20. What is a Stub Router in EIGRP?

Answer:
A stub router is used in hub-and-spoke topologies to reduce query traffic and improve stability.

🔍 Section 3: Advanced EIGRP Questions (L3/Architect Level)

21. What’s the difference between EIGRP Internal and External routes?

  • Internal: Routes learned within the same AS

  • External: Routes redistributed into EIGRP from other sources

22. How is the EIGRP metric calculated?

Answer:

Metric = 256 * (K1 * Bandwidth + K3 * Delay)

Default formula uses bandwidth and delay only.

23. What causes EIGRP stuck-in-active (SIA) state?

Answer:
A route is marked SIA when:

  • A query is sent but no reply is received

  • Neighbor fails to respond in time
    This may indicate network congestion or link failures.

24. What is the use of the variance command in EIGRP?

Answer:
Variance allows unequal-cost load balancing by permitting routes whose metric is within a multiple (variance) of the best route.

25. How can you limit EIGRP route advertisement?

  • Use route summarization

  • Configure passive interfaces

  • Implement route filtering with distribute-lists

26. What is route summarization in EIGRP?

Answer:
EIGRP supports manual summarization at any interface using:

ip summary-address eigrp <AS> <summary-ip> <mask>

27. How does EIGRP behave over non-broadcast networks like Frame Relay?

You must manually define neighbors using:

neighbor <ip> <interface>

EIGRP does not auto-discover neighbors on NBMA networks.

🔧 Section 4: EIGRP Troubleshooting & Real-World Scenarios

28. How do you verify EIGRP neighbors?

Use:

show ip eigrp neighbors

29. How do you check EIGRP routes?

Use:

show ip route eigrp

30. A router isn’t forming EIGRP neighbor adjacency. What do you check?

  • AS number

  • Subnet match

  • K-values match

  • Hello/hold timers

  • Interface status

31. How to see EIGRP topology table?

Use:

show ip eigrp topology

32. How to debug EIGRP issues?

Use:

debug eigrp packets
debug ip eigrp

Use with caution on production networks.

🔄 Section 5: EIGRP vs OSPF vs RIP

33. EIGRP vs OSPF: Key Differences

FeatureEIGRPOSPF
TypeHybridLink-State
AlgorithmDUALSPF
Protocol Number8889
Load BalancingUnequal (variance)Equal only
ConvergenceVery FastFast

34. EIGRP vs RIP

FeatureEIGRPRIP
Max Hop Count25515
ConvergenceFastSlow
VLSM SupportYesRIP v1 = No, v2 = Yes
Load BalancingUnequal & EqualEqual only

 

Top 50+ EIGRP Interview Questions and Answers (2025 Guide)

[Note: This is a continuation of the previously shared sections. Below is the remaining part of the post, expanding the question count and content to reach 3000+ words.]

🔹 Additional EIGRP Interview Questions (Continuing from Q33)

35. How do you change the hello and hold timer in EIGRP?

Answer:

interface <interface-name>
ip hello-interval eigrp <AS> <seconds>
ip hold-time eigrp <AS> <seconds>

36. What happens if K-values do not match between two EIGRP routers?

Answer:
They will not form a neighbor relationship because EIGRP requires K-values to be identical.

37. Can EIGRP advertise static and connected routes?

Answer:
Yes. Use the redistribute static and redistribute connected commands inside the EIGRP routing process.

38. What is an EIGRP summary route and its administrative distance?

Answer:
A manual summary route is a summarized network route created by an admin. It has an administrative distance of 5.

39. How to advertise a default route in EIGRP?

Answer:
Using one of the following:

ip route 0.0.0.0 0.0.0.0 <next-hop>
redistribute static

Or using:

ip default-network <network>

40. Can EIGRP run over IPv6?

Answer:
Yes, EIGRP supports IPv6. It’s configured under interface mode and uses named EIGRP configuration.

41. What is a passive interface in EIGRP?

Answer:
A passive interface prevents EIGRP from sending Hello packets on the interface, thus stopping neighbor formation, while still advertising the network.

42. How do you configure EIGRP authentication?

Answer:

interface <int>
ip authentication mode eigrp <AS> md5
ip authentication key-chain eigrp <AS> <name>
key chain <name>
 key 1
  key-string <password>

43. What is the difference between feasible distance and advertised distance?

  • Feasible Distance (FD): Total metric to reach destination

  • Advertised Distance (AD): Metric reported by the neighbor

44. How to limit the number of EIGRP prefixes sent to a neighbor?

Answer:
Use distribute-list, route-map, or prefix-list to filter and control route advertisement.

45. What is EIGRP stub configuration used for?

Answer:
Used on routers in a hub-and-spoke topology to prevent them from being queried, reducing CPU and bandwidth usage.

eigrp stub connected static

46. How do you display detailed EIGRP metrics?

Answer:

show ip eigrp topology

Also:

show ip protocols

47. What happens if there’s no feasible successor and the route goes down?

Answer:
The router enters active state, sends queries to neighbors, and waits for replies to calculate a new route.

48. What is wide metric support in EIGRP?

Answer:
It’s used in EIGRP for IPv6 or named configurations. It allows more granular metrics beyond the 32-bit limit of classic EIGRP.

49. What are some show commands used for EIGRP troubleshooting?

  • show ip eigrp interfaces

  • show ip eigrp neighbors

  • show ip eigrp topology

  • debug eigrp packet

  • show ip route eigrp

50. What are the advantages of EIGRP over other protocols?

  • Faster convergence (thanks to DUAL)

  • Lower bandwidth consumption

  • Unequal cost load balancing

  • Easy configuration

  • Route summarization

  • Compatibility with IPv6

51. What is the command to remove EIGRP from a router?

Answer:

no router eigrp <AS number>

 

🎯 Pro Tips for EIGRP Interview Success

  1. Master DUAL logic & topology table

  2. Understand metrics deeply

  3. Practice troubleshooting with real Cisco lab setups

  4. Be prepared to explain packet flows

  5. Know how EIGRP handles failures & convergence

📌 Conclusion

EIGRP remains a key topic in networking interviews, especially for Cisco-focused roles. Whether you’re applying as a Network Engineer, System Administrator, or Tech Support Specialist, these 50+ EIGRP interview questions and answers will help you showcase your understanding with clarity and confidence.

📚 Related Resources from TopInterviewQuestions.in

🏁 Final Words

Interviewers test not just your EIGRP definitions, but how you apply concepts in real-world scenarios. Practice answering out loud, build your Cisco lab, and always relate theory to troubleshooting experience.

👉 For more interview-ready questions across technologies, visit TopInterviewQuestions.in — your trusted guide for cracking tech interviews!

 

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