If you’re preparing for a network engineering interview, having a solid understanding of EIGRP (Enhanced Interior Gateway Routing Protocol) is essential. Whether you’re pursuing your CCNA, CCNP, or applying for enterprise-level roles, this guide will help you confidently answer any EIGRP-related question.
In this post, you’ll find 50 of the most frequently asked EIGRP interview questions, along with clear, accurate, and easy-to-understand answers
1. What is EIGRP and how does it work?
Answer:
EIGRP (Enhanced Interior Gateway Routing Protocol) is a hybrid routing protocol developed by Cisco. It blends the features of distance-vector and link-state protocols. EIGRP uses the DUAL (Diffusing Update Algorithm) to calculate loop-free paths and supports fast convergence, VLSM, route summarization, and unequal-cost load balancing. It exchanges routing information with neighbors via Hello packets and maintains a topology table to track all feasible routes.
2. What is the EIGRP metric formula and which K-values are used?
Answer:
EIGRP calculates its metric using this formula:
Metric = [(10⁷ / minimum bandwidth in kbps) + cumulative delay in tens of microseconds] × 256
By default, only K1 (Bandwidth = 1) and K3 (Delay = 1) are active;
K2 (Load), K4 (Reliability), and K5 (MTU) are set to 0.
These K-values can be modified using the metric weights command, but all routers must match them to form adjacencies.
3. What are Feasible Distance, Reported Distance, and the Feasibility Condition in EIGRP?
Answer:
- Feasible Distance (FD): Total cost from the local router to a destination via a successor.
- Reported Distance (RD): Cost from the neighbor to the destination.
- Feasibility Condition (FC): A backup route is valid only if its RD < FD.
This logic ensures loop-free routing by verifying that a neighbor is closer to the destination than the current path.
4. What is the difference between a Successor and Feasible Successor in EIGRP?
Answer:
- A Successor is the best next-hop router with the lowest feasible distance. It appears in the routing table.
- A Feasible Successor is a backup route that meets the Feasibility Condition and is stored in the topology table.
If the Successor fails, the Feasible Successor is immediately promoted, allowing for rapid convergence.
5. What are Hello and Hold timers in EIGRP, and how can they be modified?
Answer:
- Hello Timer: Interval at which EIGRP sends Hello packets (default: 5 sec on LAN, 60 sec on WAN)
- Hold Timer: How long a router waits to hear a Hello before declaring the neighbor down (default: 15 sec on LAN, 180 sec on WAN)
You can change them per interface using:
ip hello-interval eigrp <AS> <seconds>
ip hold-time eigrp <AS> <seconds>
6. What are Active and Passive states in EIGRP?
Answer:
- A Passive route is stable and has a known working path (Successor).
- An Active route means EIGRP is searching for a new path (because the previous one failed and no backup exists).
Active routes trigger Queries to neighbors. Prolonged Active states can lead to performance issues and are a sign of topology instability.
7. What is the purpose of Reliable Transport Protocol (RTP) in EIGRP?
Answer:
RTP is a Cisco-proprietary protocol used by EIGRP to guarantee delivery of routing messages.
It supports:
- Unicast and multicast
- Acknowledgments
- Sequencing and retransmissions
This ensures reliable delivery of critical packets like Updates and Queries, making EIGRP robust even over unreliable networks.
8. How does EIGRP support unequal-cost load balancing?
Answer:
EIGRP uses the variance command to enable unequal-cost load balancing.
If a path’s metric is ≤ (best FD × variance), it can be added to the routing table.
Example: If best FD is 1000 and variance is 2, paths with FD ≤ 2000 are used.
This allows efficient bandwidth usage across multiple links.
9. What are the five packet types used by EIGRP?
Answer:
EIGRP uses the following packet types:
- Hello – For neighbor discovery and keepalive
- Update – For sending routing information
- Query – To request alternate paths
- Reply – In response to Queries
- ACK (Acknowledgment) – For reliable packet delivery
These packet types support neighbor formation, route learning, and convergence.
10. How does EIGRP handle neighbor relationships and what conditions must match?
Answer:
To form a neighbor relationship, routers must match:
- AS number
- K-values
- Authentication (if configured)
- Primary subnet (must be on the same network)
- Non-passive interface
Neighbors are discovered using Hello packets sent to 224.0.0.10, and are listed in the show ip eigrp neighbors command.
11. What is the EIGRP topology table and how is it used?
Answer:
The topology table in EIGRP stores all routes learned from neighbors, including their Feasible Distance, Reported Distance, Successor, and Feasible Successors.
It is not used for forwarding traffic but is essential for:
- DUAL calculations
- Maintaining loop-free backup paths
- Enabling fast convergence in case of failure
Only the best (successor) route is promoted from the topology table to the routing table.
12. What is Stuck-In-Active (SIA) in EIGRP and how is it handled?
Answer:
A route becomes Stuck-In-Active (SIA) when a router queries its neighbors for a replacement path but doesn’t receive a reply within 3 minutes.
EIGRP then:
- Marks the neighbor as unreachable
- Removes it from the neighbor table
- Triggers DUAL to recalculate routes
SIA typically indicates network delays or design flaws like missing stubs or summarization.
13. What is EIGRP variance and how does it work in load balancing?
Answer:
The variance command allows EIGRP to install unequal-cost paths into the routing table.
If a route’s metric is within the range of (FD × variance), it qualifies for use.
This improves:
- Bandwidth utilization
- Redundancy
- Link failover options
By default, EIGRP installs up to 4 paths, but this can be increased with maximum-paths.
14. What is the role of the Router ID in EIGRP?
Answer:
The Router ID is a 32-bit unique identifier used in EIGRP to distinguish routers, especially in external route advertisements.
EIGRP selects it in this order:
- Manually configured (preferred)
- Highest IP on loopback
- Highest IP on physical interface
It does not affect neighbor formation, but helps in troubleshooting and route tracking.
15. How does EIGRP handle route summarization?
Answer:
EIGRP supports:
- Manual summarization via ip summary-address eigrp
- Automatic summarization at classful boundaries (disabled by default in modern IOS)
Summarization:
- Reduces routing table size
- Hides unnecessary details
- Prevents unnecessary queries when used with stubs
Use no auto-summary to disable legacy behavior.
16. What is the difference between EIGRP for IPv4 and EIGRP for IPv6?
Answer:
| Feature | EIGRP for IPv4 | EIGRP for IPv6 |
| Network command | Required | Not used |
| Interface-based config | No | Yes |
| Address type | IPv4 | IPv6 |
| Router ID | Optional | Required |
EIGRP for IPv6 is interface-driven and uses the ipv6 router eigrp mode, making configuration slightly different.
17. What are the benefits of using EIGRP Stub routers?
Answer:
Stub routers limit query propagation and reduce CPU and memory usage.
Common in hub-and-spoke designs, stub routers:
- Only advertise specific routes (connected, static, summary)
- Don’t forward transit traffic
- Prevent being queried unnecessarily
Command:
router eigrp <AS>
eigrp stub connected static
18. How can you configure authentication in EIGRP?
Answer:
EIGRP supports MD5 and SHA authentication.
To configure:
- Define a key chain:
key chain AUTH
key 1
key-string mypass
- Apply on interface:
ip authentication mode eigrp <AS> md5
ip authentication key-chain eigrp <AS> AUTH
Authentication protects against unauthorized adjacencies and spoofed updates.
19. What are some key EIGRP show commands for troubleshooting?
Answer:
Useful commands include:
- show ip eigrp neighbors – View adjacencies
- show ip eigrp topology – See known routes, FD, and route states
- show ip route eigrp – Display EIGRP routes only
- debug eigrp packets – For live packet exchange debugging
- show ip protocols – EIGRP timers, metrics, networks
These help identify neighbor issues, route loss, and SIA events.
20. What is the difference between EIGRP named mode and classic mode?
Answer:
- Classic mode: Uses router eigrp <AS> and network commands
- Named mode: Uses router eigrp <NAME>, structured sub-modes (address-family, interface, topology)
Named mode supports both IPv4 and IPv6 under one config, and offers more granular control, route filtering, and modular hierarchy.
21. How does EIGRP support IPv6 routing?
Answer:
EIGRP supports IPv6 routing using a slightly different configuration model:
- There is no network command
- Configuration is interface-based
- Requires a Router ID
- Uses the multicast address FF02::A for neighbor discovery
Example setup:
ipv6 unicast-routing
ipv6 router eigrp 10
eigrp router-id 1.1.1.1
interface g0/0
ipv6 eigrp 10
This makes EIGRP fully capable of routing in dual-stack or IPv6-only environments.
22. What is the purpose of the maximum-paths command in EIGRP?
Answer:
The maximum-paths command controls how many equal or unequal-cost paths EIGRP can install in the routing table (default is 4, maximum is 16).
It’s useful for:
- Enabling load balancing
- Improving link utilization
- Providing redundancy
Command example:
router eigrp 100
maximum-paths 6
23. How does EIGRP handle route flapping or instability?
Answer:
EIGRP efficiently handles route flapping by:
- Maintaining a topology table for quick recalculation
- Using Feasible Successors to avoid full re-convergence
- Sending incremental updates rather than full tables
- Removing unstable neighbors if they frequently go Stuck-in-Active
This makes EIGRP more stable in dynamic or unstable environments compared to protocols like RIP.
24. What is the SRTT and RTO in EIGRP neighbor relationships?
Answer:
- SRTT (Smoothed Round Trip Time): Estimated time it takes to receive an acknowledgment from a neighbor (in milliseconds)
- RTO (Retransmission Timeout): Time to wait before retransmitting an unacknowledged packet
These values are seen in show ip eigrp neighbors and help ensure reliable delivery of routing messages using RTP.
25. How does EIGRP behave differently over WAN vs LAN?
Answer:
On LANs:
- Hello: 5 sec, Hold: 15 sec
- Fast convergence and frequent neighbor updates
On WANs (e.g., Frame Relay or serial links):
- Hello: 60 sec, Hold: 180 sec
- Neighbors may need to be manually defined on NBMA networks
- Often configured with static neighbors or stubs to reduce query storms
26. How does EIGRP perform route filtering?
Answer:
EIGRP supports route filtering using:
- Distribute lists (with access-lists or prefix-lists)
- Route-maps for advanced filtering logic
- Offset-lists to manipulate metrics
- Stub routing to limit query scope
Example of filtering with prefix list:
ip prefix-list BLOCK_NET seq 5 deny 10.0.0.0/8
router eigrp 100
distribute-list prefix BLOCK_NET out
27. How can you reduce the EIGRP query scope in large networks?
Answer:
To reduce EIGRP query scope:
- Use EIGRP stub routers on spokes
- Apply manual summarization at hub routers
- Use route filtering with distribute-lists or prefix-lists
- Design a hierarchical network layout
This helps prevent Stuck-in-Active conditions and improves scalability.
28. How does EIGRP handle redistribution from other protocols?
Answer:
To redistribute routes into EIGRP:
- Define default metric using default-metric
- Use the redistribute command inside EIGRP config
- Optionally apply route-maps to control what is redistributed
Example:
router eigrp 100
default-metric 10000 100 255 1 1500
redistribute ospf 1 route-map OSPF-IN
Redistributed routes are marked as external with AD of 170.
29. What is the difference between internal and external EIGRP routes?
Answer:
| Type | Description | AD |
| Internal | Learned from routers in the same AS | 90 |
| External | Learned via redistribution or other AS | 170 |
External routes show an “EX” in the routing table and are treated with lower priority unless manually adjusted.
30. How does EIGRP scale in large enterprise networks?
Answer:
EIGRP is highly scalable due to:
- Feasible Successor logic for fast failover
- Route summarization to reduce routing table size
- Stub routing to contain queries
- Load balancing across multiple paths
- Low CPU and bandwidth usage compared to OSPF
With proper design, EIGRP can support thousands of routes efficiently.
31. What is the DUAL algorithm and how does it work in EIGRP?
Answer:
DUAL (Diffusing Update Algorithm) is the core engine behind EIGRP’s decision-making. It ensures:
- Loop-free routing at all times
- Fast convergence when topology changes
- Selection of Successors and Feasible Successors
- Use of queries and replies to discover new paths
DUAL evaluates the Feasibility Condition to validate backup paths and uses Finite State Machine (FSM) logic to track route states (Active/Passive).
32. How does EIGRP support fast convergence?
Answer:
EIGRP achieves fast convergence by:
- Maintaining Feasible Successors as precomputed backup paths
- Using DUAL to quickly react to route failures
- Sending incremental updates rather than full routing tables
- Avoiding loops through the Feasibility Condition
This allows it to switch routes in milliseconds without waiting for timers.
33. What happens during the EIGRP neighbor adjacency process?
Answer:
When two routers on the same subnet exchange Hello packets with matching parameters (AS number, K-values, timers, etc.), they form a neighbor adjacency.
Once formed:
- They exchange Update packets with full routing info
- Acknowledge using ACK packets
- Routes are placed in the topology table, then best ones in routing table
You can view neighbors with show ip eigrp neighbors.
34. Can two routers with different K-values become EIGRP neighbors?
Answer:
No, EIGRP requires matching K-values between neighbors.
If K-values differ, the routers will not form an adjacency to prevent inconsistent metric calculations, which could lead to routing loops.
35. What is the purpose of the EIGRP metric weights command?
Answer:
The metric weights command lets you manually adjust the K-values that EIGRP uses in its metric calculation.
Syntax:
metric weights tos k1 k2 k3 k4 k5
By default: metric weights 0 1 0 1 0 0
Changing these values affects route selection behavior, but should be done only with care and network-wide consistency.
36. What is the difference between Hello and Update packets in EIGRP?
Answer:
- Hello Packets: Sent periodically for neighbor discovery/maintenance; contain no routing info
- Update Packets: Sent when routes change or new adjacency is formed; contain detailed routing info
Both use multicast (224.0.0.10) and require acknowledgment (except Hello).
37. How does EIGRP handle load balancing across equal and unequal paths?
Answer:
EIGRP performs:
- Equal-cost load balancing by default (up to 4 paths)
- Unequal-cost load balancing using the variance command
Paths within (best metric × variance) range are used.
Load is distributed based on interface bandwidth, not round-robin, ensuring more efficient utilization.
38. What is an EIGRP Stub Router and why is it used?
Answer:
A stub router in EIGRP:
- Does not forward transit traffic
- Advertises only specific route types (e.g., connected, static)
- Limits the scope of queries, especially in hub-and-spoke topologies
Use:
router eigrp 100
eigrp stub connected static
This reduces CPU/memory usage and enhances stability in large networks.
39. What are the key differences between EIGRP and OSPF?
Answer:
| Feature | EIGRP | OSPF |
| Type | Cisco proprietary (hybrid) | Open standard (link-state) |
| Metric | Composite (BW, Delay) | Cost (based on BW) |
| Convergence | Very fast (DUAL) | Fast (LSDB + SPF) |
| Load Balance | Equal & unequal | Equal only |
| Areas | Not used | Area-based hierarchy |
| Configuration | Simple (fewer steps) | More complex (areas, DR/BDR) |
EIGRP is easier in Cisco-only networks; OSPF is better for multi-vendor compatibility.
40. What are some design best practices when using EIGRP?
Answer:
- Use manual summarization at boundaries
- Implement stub routers in hub-and-spoke topologies
- Keep K-values consistent across routers
- Disable auto-summary unless required
- Plan query boundaries to avoid Stuck-in-Active issues
- Monitor with show ip eigrp topology and show ip eigrp neighbors
- Use authentication for security in enterprise networks
These practices ensure a scalable, loop-free, and reliable EIGRP deployment.
41. What is the impact of summarization on EIGRP query scope?
Answer:
Route summarization reduces the query scope by:
- Creating a summary route that hides more specific routes
- Preventing the router from sending queries about subnets it no longer advertises
This limits the spread of EIGRP queries, improves convergence time, and reduces the risk of Stuck-in-Active (SIA) states.
42. Can EIGRP operate across discontiguous networks?
Answer:
Yes — EIGRP supports classless routing, including discontiguous subnets, provided:
- Auto-summary is disabled using no auto-summary
- Proper summarization and redistribution strategies are used
Discontiguous network handling makes EIGRP suitable for complex enterprise topologies.
43. How does EIGRP handle routing loops?
Answer:
EIGRP prevents routing loops through:
- The Feasibility Condition (FC)
- Use of Successor and Feasible Successor logic
- The DUAL algorithm, which ensures a router only uses a path if the neighbor is closer to the destination
Unlike RIP, EIGRP does not rely on split horizon or hold-down timers alone.
44. What does the Passive keyword mean in the EIGRP topology table?
Answer:
In the show ip eigrp topology output:
- Passive means the route is stable and ready for forwarding
- Active means EIGRP is searching for a new path due to route failure
A route should always be Passive under normal conditions.
45. How does EIGRP support route redistribution from OSPF or RIP?
Answer:
EIGRP can redistribute routes from other protocols like OSPF or RIP using:
router eigrp 100
default-metric 10000 100 255 1 1500
redistribute ospf 1
This assigns a composite metric for EIGRP to evaluate routes. You can also use route-maps and tagging for better control and filtering.
46. What are some causes of EIGRP neighbor adjacency failure?
Answer:
Common causes include:
- Mismatched AS numbers
- K-values not matching
- Different Hello/Hold timers
- Passive interfaces
- Authentication mismatches
- Subnets not matching or ACLs blocking multicast (224.0.0.10)
Troubleshooting can be done with:
show ip eigrp neighbors
debug eigrp packets
47. What is the default administrative distance (AD) for EIGRP routes?
Answer:
| Route Type | Administrative Distance |
| Internal EIGRP | 90 |
| External EIGRP | 170 |
| Summary Route | 5 |
This value helps routers prefer EIGRP routes over other protocols (like RIP with AD 120), unless manually changed.
48. What is the function of the redistribute static command in EIGRP?
Answer:
The redistribute static command allows static routes to be injected into EIGRP.
You must assign a default metric beforehand or within the command.
Example:
router eigrp 100
default-metric 10000 100 255 1 1500
redistribute static
This is useful for routing to external networks or simplified configurations.
49. What is the multicast address used by EIGRP and why?
Answer:
EIGRP uses 224.0.0.10 (IPv4) and FF02::A (IPv6) for sending Hello, Update, Query, and Reply packets.
These addresses:
- Limit traffic to EIGRP-enabled routers only
- Reduce broadcast overhead
- Help with efficient neighbor discovery and routing updates
50. Is EIGRP a classless or classful protocol?
Answer:
EIGRP is a classless routing protocol — it supports:
- VLSM (Variable Length Subnet Masks)
- CIDR (Classless Inter-Domain Routing)
- Manual summarization at any bit boundary
Although older versions had auto-summary enabled by default, that can and should be turned off using:
no auto-summary
This ensures proper handling of discontiguous and complex subnetting schemes.
Conclusion:
Whether you’re heading into a CCNP-level interview or refreshing your networking knowledge, mastering EIGRP gives you a serious advantage. These questions cover everything from basic concepts to advanced troubleshooting, making this the only EIGRP Q&A guide you’ll need.
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