[2024] CCNA Interview Questions on EIGRP

Enhanced Interior Gateway Routing Protocol (EIGRP) is a key dynamic routing protocol used in Cisco networks to efficiently manage and optimize routing decisions. As a hybrid protocol, EIGRP combines the best features of both distance-vector and link-state protocols, offering a robust solution for managing routing in large and complex network environments. For those preparing for CCNA interviews, understanding EIGRP is crucial. This article covers essential EIGRP interview questions that can help you showcase your expertise and prepare effectively for your next interview.

1. What is EIGRP, and how does it differ from other routing protocols?

Answer: EIGRP, or Enhanced Interior Gateway Routing Protocol, is a Cisco proprietary routing protocol designed to improve upon older distance-vector protocols like RIP and offer advanced features found in link-state protocols. Unlike RIP, which uses hop count as a metric, EIGRP uses a composite metric based on bandwidth, delay, load, and reliability. EIGRP also features rapid convergence and supports both IPv4 and IPv6.

2. Can you explain the EIGRP metric calculation?

Answer: EIGRP calculates its metric based on a composite of several factors: bandwidth, delay, load, and reliability. The formula used is: $\text{Metric} = \left( \frac{10^7}{\text{Minimum Bandwidth}} + \text{Total Delay} \right) \times 256$

• Minimum Bandwidth: The lowest bandwidth of the path.
• Total Delay: The cumulative delay of the path (in microseconds).

3. What is the purpose of the EIGRP router ID, and how is it determined?

Answer: The EIGRP router ID is a 32-bit value used to uniquely identify a router in an EIGRP network. It is important for maintaining the consistency and reliability of routing information. The router ID is determined in the following order:

1. Manually Configured Router ID using the eigrp router-id command.
2. Highest IP Address on a Loopback Interface if no router ID is manually set.
3. Highest IP Address on a Physical Interface if no loopback interfaces are present.

4. How does EIGRP use the Diffusing Update Algorithm (DUAL)?

Answer: DUAL is the algorithm EIGRP uses to ensure loop-free and efficient routing updates. DUAL calculates the shortest path to each destination and maintains a topology table of all possible routes. It uses this information to determine the best route and backup routes, facilitating rapid convergence and minimizing routing loops.

5. What are the different EIGRP message types?

Answer: EIGRP uses several types of messages to communicate between routers:

• Hello: Used to discover and maintain neighbor relationships.
• Update: Used to send routing information to neighbors.
• Query: Used to request routing information from neighbors if there are no routes available.
• Reply: Used to respond to a Query message with routing information.

6. What is EIGRP's topology table, and what information does it contain?

Answer: The EIGRP topology table stores information about all the routes advertised by EIGRP neighbors. It contains the following information:

• Destination Network: The network being advertised.
• Successor: The primary route to the destination.
• Feasible Successor: A backup route that can be used if the primary route fails.
• Metric Information: Details on the path’s metric, including bandwidth, delay, load, and reliability.

7. What is the significance of EIGRP's feasible distance (FD) and reported distance (RD)?

Answer:

• Feasible Distance (FD): The total metric of the best route to a destination as calculated by the local router.
• Reported Distance (RD): The metric reported by a neighbor router for a particular route.

The FD is used to determine the best route, while the RD helps in identifying feasible successors.

8. How does EIGRP handle route summarization?

Answer: Route summarization in EIGRP is used to aggregate multiple IP addresses into a single summary address. This helps in reducing the size of routing tables and improving network performance. EIGRP automatically summarizes routes at classful boundaries by default, but manual summarization can be configured using the ip summary-address eigrp command.

9. What are the main differences between EIGRP and OSPF?

Answer: EIGRP and OSPF are both routing protocols but differ in several aspects:

• EIGRP: Hybrid protocol combining distance-vector and link-state features; uses DUAL algorithm; supports both IPv4 and IPv6; faster convergence.
• OSPF: Link-state protocol; uses Dijkstra’s algorithm; primarily used for large enterprise networks; uses area-based hierarchy to optimize routing.

10. How does EIGRP achieve rapid convergence?

Answer: EIGRP achieves rapid convergence through its DUAL algorithm, which maintains both primary and backup routes. Upon detecting a topology change, DUAL quickly recalculates the best path using the pre-computed backup routes, thus minimizing the time taken to converge and ensuring stable network operations.

11. What is EIGRP's hold time and hello interval?

Answer:

• Hello Interval: The time interval at which EIGRP Hello packets are sent to maintain neighbor relationships. The default is 5 seconds.
• Hold Time: The time a router waits for a Hello packet from a neighbor before declaring the neighbor as unreachable. The default is 15 seconds.

12. How do you configure EIGRP on a router?

Answer: To configure EIGRP on a router, use the following commands:

1. Enter EIGRP router configuration mode:
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   Router(config)# router eigrp [AS_NUMBER]
2. Define the network to be included in EIGRP:
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   Router(config-router)# network [NETWORK_ADDRESS] [WILDCARD_MASK]

13. What is EIGRP's administrative distance, and how does it compare to other protocols?

Answer: The administrative distance (AD) of EIGRP is 90. This value indicates the trustworthiness of the EIGRP routes compared to other routing protocols. Lower AD values denote higher trust. For comparison, RIP has an AD of 120, and OSPF has an AD of 110.

14. Can you explain the EIGRP neighbor states?

Answer: EIGRP neighbor states are used to describe the relationship between EIGRP routers:

• Down: No Hello packets received.
• Init: Hello packets received but no acknowledgment.
• Up: Hello packets received and acknowledgment exchanged; routers are neighbors.
• Exchange: Routers are exchanging routing information.
• Loading: Routers are loading the routing information into their tables.

15. How does EIGRP handle route redistribution?

Answer: EIGRP can redistribute routes from other routing protocols into its own routing table. This is done using the redistribute command within the EIGRP router configuration. For example, to redistribute routes from OSPF into EIGRP, use:

16. What is EIGRP's "K" value and how does it affect the metric calculation?

Answer: EIGRP uses a set of "K" values to determine the metric for routing decisions. These values represent different factors in the metric calculation:

• K1: Bandwidth
• K2: Load
• K3: Delay
• K4: Reliability
• K5: MTU (Maximum Transmission Unit)

The default K values are set to 1 for bandwidth and delay, and 0 for load, reliability, and MTU. Adjusting these values can affect how EIGRP calculates the metric for routes, influencing routing decisions.

17. What is the purpose of EIGRP's stub routing?

Answer: Stub routing in EIGRP is used to simplify routing in smaller or edge routers by reducing the amount of routing information they need to handle. A stub router will not accept or advertise routes learned from other routers, which helps in reducing overhead and improving efficiency in certain network topologies.

18. How do you configure EIGRP authentication, and why is it important?

Answer: EIGRP authentication ensures that routing updates are exchanged only between trusted routers, enhancing security. To configure EIGRP authentication:

1. Configure an authentication key on the router:
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   Router(config)# interface [INTERFACE_NAME] Router(config-if)# ip authentication mode eigrp [AS_NUMBER] md5 Router(config-if)# ip authentication key-chain eigrp [AS_NUMBER] [KEY_CHAIN_NAME]
2. Define the key chain:
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   Router(config)# key-chain [KEY_CHAIN_NAME] Router(config-keychain)# key [KEY_ID] Router(config-keychain-key)# key-string [PASSWORD]

19. What is the purpose of the passive-interface command in EIGRP?

Answer: The passive-interface command is used to prevent EIGRP from sending Hello packets or receiving routing updates on a specific interface. This is useful for interfaces where routing updates are not needed, such as those connected to end devices or where security is a concern.

20. How does EIGRP handle route filtering?

Answer: EIGRP allows route filtering through the use of distribute lists and prefix lists. Distribute lists control which routes are advertised to or accepted from neighbors, while prefix lists provide more granular control over which routes are filtered based on IP address prefixes.

21. What are the EIGRP metric weights, and how do they affect routing?

Answer: EIGRP metric weights are configurable values that influence how different factors (bandwidth, delay, load, reliability) are considered in the metric calculation. By adjusting these weights, network administrators can influence the metric and, consequently, the selection of the best path for routing.

22. Explain the concept of EIGRP route redistribution and its configuration.

Answer: Route redistribution involves sharing routing information between different routing protocols. In EIGRP, you can redistribute routes from other protocols into EIGRP using the redistribute command. Example for redistributing static routes:

This allows EIGRP to include routes from static routes or other routing protocols in its routing table.

23. What is EIGRP's EIGRP metric and how does it impact network design?

Answer: The EIGRP metric is a composite value that determines the cost of a route. It is calculated based on factors such as bandwidth, delay, load, and reliability. Network design should consider EIGRP metrics to ensure optimal routing paths and performance, especially in complex networks with varying link qualities.

24. How does EIGRP handle route summarization at the interface level?

Answer: Route summarization at the interface level in EIGRP can be configured to aggregate multiple routes into a single summary route. This reduces the size of the routing table and improves network performance. Example command:

25. What is the impact of EIGRP on network scalability and how can it be managed?

Answer: EIGRP supports network scalability through its efficient use of bandwidth and fast convergence times. To manage scalability, consider implementing route summarization, EIGRP stub routing, and distribution lists to control routing information and optimize network performance.

Conclusion

EIGRP is a versatile and efficient routing protocol that plays a crucial role in modern network infrastructures. Mastery of EIGRP concepts and configurations is essential for CCNA candidates to demonstrate their routing expertise. By understanding and preparing for these common EIGRP interview questions, you'll be well-equipped to showcase your knowledge and tackle any EIGRP-related questions that come your way in a CCNA interview.