🎯 Part 5 of 5 · Interview Q&A Master

EIGRP Module A · Enterprise Routing Master Series

50 Expert Q&A + 15 Interactive Quiz Questions

The complete CCNP/CCIE interview preparation toolkit: 15 quick-fire facts, 15 one-sentence concept answers, 20 scenario-based troubleshooting questions, and a 15-question interactive quiz with scoring and replay.

👤 Author: Hrishi Pradhan

🎯 Level: Basic to Advanced

🎤 Q&As: 50 Expandable

🎓 Quiz: 15 MCQ (Replayable)

📌 Section A — Quick-Fire Facts (One-Word Answers)

15 rapid questions with single-word or short-phrase answers

15 Questions

📌 How to Use This Section

Click the ▼ arrow next to any question to reveal the answer. These are single-word or short-phrase answers that interviewers expect you to know instantly.

1 What IP protocol number does EIGRP use for encapsulation?

88 — EIGRP rides directly on IP without TCP or UDP wrappers.

2 What multicast address does EIGRP use for Hello packets?

224.0.0.10 — the well-known "EIGRP Routers" multicast address.

3 What is the default Hello timer for EIGRP on Ethernet links?

5 seconds — with a corresponding Hold timer of 15 seconds.

4 What is the default Hold timer for EIGRP on Ethernet links?

15 seconds — three times the Hello interval for buffer margin.

5 What is the Administrative Distance of Internal EIGRP?

90 — preferred over OSPF (110) and RIP (120).

6 What is the Administrative Distance of External EIGRP?

170 — applied to redistributed routes.

7 What does RTP stand for in EIGRP?

Reliable Transport Protocol — Cisco-proprietary reliable delivery mechanism.

8 What does DUAL stand for?

Diffusing Update Algorithm — EIGRP's loop-free path calculation engine.

9 How many times does EIGRP retry an unacknowledged packet via unicast?

16 times — after which the neighborship flaps.

10 What does SRTT stand for?

Smooth Round Trip Time — average time between sending and receiving ACK.

11 What does RTO stand for?

Retransmission Timeout — max wait before retry (default up to 5000 ms).

12 What is the maximum configurable variance value?

128 — with a default of 1.

13 Which EIGRP message type uses no acknowledgment?

Hello — it is unreliable, sent via multicast every 5 seconds.

14 What is the Administrative Distance of an EIGRP summary route?

5 — used for the automatically generated Null0 discard route.

15 What is the default maximum-path value in EIGRP?

4 — but can be raised up to 32 in IOS v15 and later.

📖 Section B — Concept Clarity (One-Sentence Answers)

15 concept questions with concise single-sentence answers

15 Questions

📌 How to Use This Section

These questions test your conceptual understanding of EIGRP. Each answer is one sentence — the type of answer expected in a real interview setting.

1 What is the difference between Neighborship and Adjacency in EIGRP?

Neighborship is discovered via Hello packets and confirms one-way reachability, while Adjacency is formed after a successful Null Update exchange, confirming bidirectional reliability required for route exchange.

2 Why is EIGRP considered an "advanced distance-vector" protocol?

Because it uses distance-vector logic (routes learned from neighbors) but adds link-state features like a topology table, neighbor tables, and DUAL for loop-free convergence.

3 What is the purpose of a Null Update in EIGRP?

It is a blank update packet that verifies bidirectional reachability between neighbors before full adjacency is established.

4 Why does EIGRP use a Hold timer of 3x the Hello timer?

To provide a buffer margin that allows other control protocols (STP, DTP, CDP) to share bandwidth without causing premature neighbor flaps.

5 What is the purpose of the RTP in EIGRP?

RTP ensures reliable delivery of EIGRP updates, queries, and replies through acknowledgments and retransmissions, since IP itself is connectionless.

6 Why do EIGRP Hello and Hold timers not need to match on both sides?

Because each router uses its own timers independently — however, mismatches can cause premature neighbor flaps if a neighbor's hold timer expires before hellos arrive.

7 What does a high Q-Count indicate in EIGRP neighbor statistics?

A backlog of pending unacknowledged packets, which signals congestion, packet loss, or a slow neighbor that is not acknowledging updates.

8 Why is static unicast neighborship used instead of dynamic multicast in some designs?

For security-conscious environments where multicast traffic is restricted and engineers want to control exactly which peers can form adjacencies.

9 Why can't static and dynamic neighborship mix on the same link?

Because static uses unicast Hellos while dynamic uses multicast 224.0.0.10 — the two do not interoperate, so neither side ever hears the other.

10 What happens to a passive interface in EIGRP?

It stops sending and receiving EIGRP Hello packets entirely, preventing any neighborship from forming on that interface.

11 Why is EIGRP considered more bandwidth-efficient than RIP?

Because EIGRP uses triggered updates and reliable transport instead of periodic full-table broadcasts.

12 What is the significance of the EIGRP AS number?

It must match on both routers for a neighborship to form, and it identifies the routing domain that the EIGRP process belongs to.

13 Why does EIGRP use a topology table in addition to the routing table?

The topology table stores all learned paths including feasible successors and alternates, allowing instant failover without recomputation.

14 What is the purpose of the Feasibility Condition in EIGRP?

It prevents routing loops by ensuring that a backup path's Reported Distance is strictly less than the primary successor's Feasible Distance.

15 Why is EIGRP considered a "hybrid" routing protocol?

Because it combines distance-vector logic (routes learned from neighbors) with link-state features (topology table, triggered updates, fast convergence).

🧠 Section C — Scenario-Based Q&A

20 real-world troubleshooting scenarios with detailed answers

20 Scenarios

📌 How to Use This Section

These are the exact types of scenario-based questions asked in CCIE labs and senior engineer interviews. Click the ▼ arrow to reveal the detailed solution.

1 You notice EIGRP adjacency between two routers is stuck in the INIT state. What are the most likely causes and how do you troubleshoot?

Common Causes:

Hello packets are reaching one side but not the other (one-way communication)
Unicast reverse path forwarding (uRPF) dropping replies
ACL blocking EIGRP (protocol 88 or multicast 224.0.0.10)
MTU or fragmentation issues on the path

Troubleshooting Steps:

show ip eigrp neighbors
debug eigrp packets hello
show ip interface | include access list
show ip access-lists

Fix: Verify bidirectional hello reachability, remove blocking ACLs, and disable uRPF if it's dropping asymmetric traffic.

2 Two EIGRP routers have their K-values mismatched, and the adjacency keeps flapping. How do you resolve this in production?

Root Cause: One router has custom metric weights (e.g., metric weights 0 1 1 1 1 1) while the other uses defaults.

Fix — Align to Default on BOTH routers:

router eigrp 1234
 metric weights 0 1 0 1 0 0

! Verify
show ip protocols | include K-values

Production Tip: Always apply K-value changes during a maintenance window — clearing the EIGRP process causes routing disruptions.

3 An EIGRP neighbor shows Q-Count consistently above 5. What is this telling you, and how do you fix it?

Meaning: High Q-Count indicates pending unacknowledged packets — the local router is waiting for ACKs that aren't arriving.

Possible Causes: Congested link, slow neighbor CPU, packet loss, or MTU/fragmentation issues.

Diagnostic Commands:

show ip eigrp neighbors detail
show interface | include error|drop|CRC
show processes cpu | include EIGRP

Fix: Reduce congestion, verify MTU consistency, check for duplex mismatches, and ensure the neighbor CPU isn't overloaded.

4 You deployed static unicast neighborship on one router, but the other router is using dynamic multicast. What happens, and how do you fix it?

What Happens: The adjacency never forms. Static unicast and dynamic multicast are incompatible — neither side sends Hellos the other can receive.

Verification:

show ip eigrp neighbors
! Shows empty peer list
! Syslog: Neighbor down: Static and dynamic configurations incompatible

Fix: Make both sides consistent — either both static OR both dynamic.

5 A network engineer changed the Hello timer to 3 seconds on one side but forgot the other. What symptoms will you see?

Symptoms:

Adjacency may flap intermittently
Neighbor occasionally expires on the side with the default 15s Hold timer
Syslog: Neighbor down: holding time expired

Why: The router with 3s Hello sends faster, but the other side's hold timer is still 15s. If even one Hello is delayed by more than 15s, the neighbor is declared dead.

Fix: Align timers on both sides. Recommended ratio: Hold = 3 × Hello.

6 You see duplicate router IDs in EIGRP logs. What is the impact, and what's the fix?

Impact: Routes from the duplicated RID are silently discarded. Partial reachability failures occur with no obvious correlation to a single event.

Syslog:

%DUAL-4-DUP_RID: Duplicate router ID 1.1.1.1 detected from 10.1.12.2

Fix:

router eigrp 1234
 eigrp router-id 10.1.1.99
clear ip eigrp 1234 neighbors *

7 A new router is added to the network but fails to form EIGRP adjacency with existing routers. You notice the AS number is correct. What else should you check?

Checklist:

Subnet match: Both interfaces must be in the same IP subnet
K-values: Must match on both sides
Authentication: Key IDs and key strings must match if MD5 is used
Passive-interface: Ensure the new interface is NOT passive
MTU: Mismatched MTU can prevent Null Update exchange
ACLs: Check for filters on protocol 88 or 224.0.0.10

8 During a link failure, you notice EIGRP routes take 3 minutes to converge. What is causing this, and how do you fix it?

Root Cause: No feasible successor exists, so the route enters Active state and EIGRP sends queries. If neighbors don't respond within 3 minutes (default active-timer), SIA occurs.

Fix Options:

Deploy stub routing on edge routers to limit query scope
Configure summarization to confine queries
Tune active-timer (rare — usually the network design is the issue)

router eigrp 1234
 eigrp stub connected summary

9 You have EIGRP running over a WAN link with MTU 1400, but the peer has MTU 1500. What issue will you face, and how do you fix it?

Issue: EIGRP has no MTU-checking mechanism like OSPF, so adjacency will still form. But Null Updates larger than 1400 bytes may be dropped, causing intermittent adjacency flaps.

Fix: Align MTUs on both sides:

interface GigabitEthernet0/0
 ip mtu 1400

Note: Unlike OSPF, EIGRP doesn't get stuck in EXSTART; it just silently drops large packets.

10 A router is configured with passive-interface default globally, but you need to enable EIGRP only on one specific interface. How do you do it?

Configuration:

router eigrp 1234
 passive-interface default
 no passive-interface GigabitEthernet0/0

Why This Works: passive-interface default stops Hellos on all interfaces, then no passive-interface selectively enables the one you want. This is the recommended security best practice.

11 You see the log message "Neighbor down: Authentication failure." Walk through the diagnosis.

Diagnostic Steps:

show key chain EIGRP-KEY
show run interface GigabitEthernet0/0 | include authentication
show ip eigrp neighbors

Common Causes:

Mismatched key-string (typo is most common)
Different key IDs on each side
Authentication mode mismatch (MD5 vs clear-text)
Key-chain not applied to interface

Fix: Ensure key ID, key-string, and mode match exactly on both sides.

12 You configure a static neighbor on only one side of a link. What happens?

Result: Adjacency fails to form. The static side sends unicast Hellos to the peer, but the peer sends multicast Hellos that the static side ignores.

Syslog:

%DUAL-5-NBRCHANGE: Neighbor 10.1.12.2 is down: Static and dynamic neighbor configurations are incompatible

Fix: Configure the neighbor command on BOTH sides of the link.

13 You want to restrict EIGRP to only run on a specific subnet. How do you do it cleanly?

Best Practice Configuration:

router eigrp 1234
 passive-interface default
 no passive-interface GigabitEthernet0/0
 network 10.1.12.0 0.0.0.255

Why This Works: The network statement activates EIGRP on matching interfaces, but passive-interface default keeps Hellos silenced everywhere else. Only the specified interface participates.

14 You notice EIGRP neighbor count is fine, but routes are missing from the routing table. What could be wrong?

Possible Causes:

Route filtering: A distribute-list, prefix-list, or route-map is blocking the routes
Administrative Distance: A route with lower AD (e.g., static) is winning
Stub configuration: A stub router may not be advertising connected/static routes without proper keywords
Duplicate router ID: Routes silently discarded

Diagnostic:

show ip eigrp topology
show ip protocols
show ip route | include [prefix]

15 You need to migrate from dynamic multicast to static unicast on a live link without dropping routes. What is the correct sequence?

Recommended Sequence:

Configure static neighbor commands on BOTH routers — do NOT remove dynamic config yet
Verify static adjacency forms (it will coexist briefly)
Remove any passive-interface or multicast-specific settings if they were blocking
Monitor with show ip eigrp neighbors to confirm stable adjacency

Important: EIGRP does not allow static and dynamic on the same interface — you must plan a brief window where both sides are cut over.

16 A router's EIGRP process is cleared with clear ip eigrp neighbors *. What happens to routes, and how fast do they reconverge?

Immediate Effect: All EIGRP neighborships are torn down. Routes learned from EIGRP are removed from the routing table.

Reconvergence: Typically within 10–30 seconds — Hellos re-exchange, Null Updates verify bidirectionality, and topology tables repopulate.

Production Tip: Never run this command casually on a production core router. Use during maintenance windows or apply clear ip eigrp neighbors [specific-ip] for single neighbors.

17 You see the log "Ignoring K-value mismatch". What specific steps do you take to restore the adjacency?

Step-by-Step Fix:

! On both routers:
show ip protocols | include K-values
! Note current values

router eigrp 1234
 metric weights 0 1 0 1 0 0
! Align to default

! Verify
show ip eigrp neighbors

Warning: If K-values were custom because of a specific QoS or path-engineering design, do NOT blindly reset to default — align them to the intended values.

18 An interface shows "EIGRP enabled" but the neighbor table is empty. What hidden issue could exist?

Hidden Causes to Check:

Passive interface: The interface is passive globally
AS mismatch: Local router is in AS 100, peer is in AS 200
Subnet mismatch: Interfaces on different subnets
Unicast RPF: Reverse path forwarding blocking the return path
Layer 2 issue: Trunk VLAN mismatch or STP blocking

show ip eigrp interfaces
show ip eigrp interfaces detail | include passive
show ip protocols | include AS

19 What is the safest way to perform EIGRP route filtering without breaking existing adjacencies?

Recommended Approach:

Build the filter ACL or prefix-list first, verify syntax with show ip prefix-list
Apply to a test interface first (in a lab), then production
Apply distribute-list ... in at the EIGRP process level (inbound filtering is safer than outbound — no risk of breaking downstream routers)
Verify with show ip eigrp topology that routes are filtered as expected

ip prefix-list FILTER permit 10.1.0.0/16 ge 24 le 28

router eigrp 1234
 distribute-list prefix FILTER in GigabitEthernet0/0

20 Your EIGRP adjacency between two routers was stable, but after adding an ACL for security, it dropped. What ACL-related issue is most likely?

Most Likely Cause: The ACL is blocking EIGRP's protocol 88 traffic or the multicast address 224.0.0.10. Even well-intentioned security ACLs commonly forget EIGRP.

Fix: Add explicit permit statements at the TOP of the ACL:

ip access-list extended SECURITY_ACL
 permit eigrp any any
 permit ip 224.0.0.10 0.0.0.0 any
 ! ... rest of ACL

Verification:

show ip eigrp neighbors
show ip access-lists SECURITY_ACL

🎯 Section D — Interactive Quiz

15 MCQs · Live scoring · Randomized on each attempt

Interactive

📌 How to Use This Quiz

Answer 15 multiple-choice questions. Get instant scoring. Retake the quiz any number of times — questions and answer options are shuffled on each attempt.

Question 1 of 15

Attempt: 1

Score: 0

Answered: 0 / 15

🎉 EIGRP Module A Complete!

You have mastered all 5 parts of EIGRP Module A — from theory to interview preparation.

🔮 Up Next in the EIGRP Master Series

EIGRP Module B — DUAL Algorithm & Path Selection

Go deeper into EIGRP's computational core. Module B covers the Diffusing Update Algorithm in full detail: Successor vs Feasible Successor selection, the Feasibility Condition, topology table metrics (FD, CD, RD), query and reply propagation, and hands-on troubleshooting of Stuck-In-Active (SIA) failure states.

📘 4 Theory Sections

🧪 5 Progressive Labs

🎤 50+ Interview Q&As

⏱ ~2 hours Estimated

⏳ Coming Soon — In Development