Part 3 of 5 · Troubleshooting Vault

EIGRP Module B · Enterprise Routing Master Series

Troubleshooting Vault & Production Error Reference

Six real-world EIGRP DUAL and SIA failures with authentic syslog messages, root cause analysis, and production-hardened fixes. Each error is documented from live field experience — use this as your on-call reference when topology and query mechanics fail in production.

Author: Hrishi Pradhan

Level: Basic to Advanced

Errors: 6 Production Scenarios

Fixes: Verified CLI

Troubleshooting Vault

Six production failures with authentic syslog messages and hardened fixes

Critical Errors

How to Use This Vault

Each error below includes the exact syslog message you will see in production, the root cause, the business impact, and the verified CLI fix. Match your syslog to the error heading to jump to the resolution.

1 Stuck-In-Active (SIA) Event Critical

Syslog Message

%DUAL-3-SIA: Route 10.1.10.0/24 stuck-in-active state in IP-EIGRP(0) 1234. Cleaning up

Root Cause

A neighbor did not reply to a Query within the Active Timer (default 3 minutes). DUAL declared the neighbor Stuck-In-Active and tore down the adjacency. Common cause: a large query domain where queries cascade across many routers before a reply is returned.

Business Impact

Neighbor adjacency resets, causing route reconvergence
Cascading SIA events across the network if design is poor
Route blackholes during the reconvergence window
Repeated SIA events create ongoing instability

Production Fix

! Step 1: Identify the query bottleneck
show ip eigrp topology active
show ip eigrp events | include SIA

! Step 2: Confine query scope on edge routers
router eigrp 1234
 eigrp stub connected summary

! Step 3: Summarize at boundary routers
interface GigabitEthernet0/0
 ip summary-address eigrp 1234 10.16.0.0 255.255.240.0

! Step 4: Verify topology after fix
show ip eigrp topology | include Active

2 Missing Feasible Successor After Link Failure Critical

Syslog Message

%DUAL-5-NBRCHANGE: EIGRP-IPv4 1234: Neighbor 10.1.12.2 (Gi0/1) is down: interface down
%DUAL-5-NBRCHANGE: EIGRP-IPv4 1234: Neighbor 10.1.13.2 (Gi0/2) is down: interface down

Root Cause

The primary successor failed, and no feasible successor existed for the destination. The route entered Active state and no query reply was returned, so the route was removed entirely. This typically happens when the secondary path fails the Feasibility Condition (RD ≥ FD) and cannot be promoted.

Business Impact

Route removed from routing table — destination unreachable
Users lose connectivity to affected subnets
Active queries flood the network looking for alternatives
If no path exists, the destination becomes permanently unavailable

Production Fix

! Step 1: Verify the topology table state
show ip eigrp topology
! Look for "0 successors" or Active state entries

! Step 2: Check all paths (including those failing Feasibility)
show ip eigrp topology all-links

! Step 3: If the alternate path is valid, adjust the metric
! (usually by tuning delay on the preferred interface)
interface GigabitEthernet0/1
 delay 10
! This lowers the metric and may bring the path within Feasibility

! Step 4: Verify recovery
show ip eigrp topology | include Successor
show ip route | include via

3 Query Storm from Improper Stub Configuration Critical

Syslog Message

%DUAL-5-NBRCHANGE: EIGRP-IPv4 1234: Neighbor 10.1.12.2 (Gi0/1) is down: peer not stub-compatible
%DUAL-5-NBRCHANGE: EIGRP-IPv4 1234: Neighbor 10.1.13.2 (Gi0/2) is down: peer not stub-compatible

Root Cause

A router was configured as EIGRP stub but its neighbors still expect it to forward queries. When the neighbor sends a query that cannot be answered by the stub, the neighbor tears down the adjacency. This occurs when stub was deployed without aligning the design across the network.

Business Impact

Adjacency drops unexpectedly after stub config change
Routes through the affected neighbor are removed
Query storms from neighbors trying to find alternatives
Requires careful design coordination, not casual config change

Production Fix

! Step 1: Verify stub status on both sides
show ip protocols | section Stub

! Step 2: Correct stub configuration to match design intent
router eigrp 1234
 no eigrp stub
 eigrp stub connected summary
! Or, if this router SHOULD forward queries:
 no eigrp stub

! Step 3: Verify neighbor adjacencies return
show ip eigrp neighbors
show ip eigrp neighbors detail | include stub

4 Flapping Active/Passive Transitions Warning

Syslog Message

%DUAL-5-NBRCHANGE: EIGRP-IPv4 1234: Neighbor 10.1.12.2 (Gi0/1) is up: new adjacency
%DUAL-5-NBRCHANGE: EIGRP-IPv4 1234: Neighbor 10.1.12.2 (Gi0/1) is down: retry limit exceeded
%DUAL-5-NBRCHANGE: EIGRP-IPv4 1234: Neighbor 10.1.12.2 (Gi0/1) is up: new adjacency

Root Cause

Routes transition rapidly between Active and Passive states because of an unstable underlying link. Each time the link flaps, DUAL re-enters Active state, sends Queries, and tries to find a new successor. If the link is unstable (physical errors, duplex mismatch, or flapping router CPU), the cycle repeats.

Business Impact

Routes constantly change next-hop, causing jitter and packet loss
Applications experience intermittent failures
CPU spikes from rapid topology recalculations
Long-term instability can mask other design problems

Production Fix

! Step 1: Identify the unstable link
show interfaces Gi0/1 | include error|drop|reset|flap
show log | include UPDOWN

! Step 2: Fix physical layer issues
! - Verify cable integrity
! - Check duplex (should be full-duplex)
! - Check speed negotiation
! - Verify SFP / port hardware

! Step 3: Verify after fix
show ip eigrp topology active
! Should show empty output after stabilization

! Step 4: If link is stable but flapping continues, adjust timers
interface Gi0/1
 ip hello-interval eigrp 1234 5
 ip hold-time eigrp 1234 15

5 Unreachable Feasible Successor (Feasibility Failed) Warning

Syslog Message

%DUAL-5-NBRCHANGE: EIGRP-IPv4 1234: Neighbor 10.1.13.2 (Gi0/2) is up: new adjacency
%DUAL-6-DUAL_STATE: Route 10.1.10.0/24 became Active -- no feasible successor
%DUAL-5-NBRCHANGE: EIGRP-IPv4 1234: Neighbor 10.1.13.2 (Gi0/2) is down: holding time expired

Root Cause

A backup path exists in the topology table but it fails the Feasibility Condition (RD ≥ FD). Because it is not a valid Feasible Successor, DUAL cannot promote it during failover and enters Active state instead. If the neighbor is also unreachable, the route is removed entirely.

Business Impact

Route moves from Passive to Active unnecessarily
Extended convergence time (up to 3 minutes for SIA)
Packet loss during Active state window
Query propagation across the network for a route with an existing (but non-qualifying) alternate

Production Fix

! Step 1: Verify the topology state
show ip eigrp topology all-links
! Identify paths failing the Feasibility Condition

! Step 2: Determine why RD >= FD
! Common causes:
!   - Offset-list inflating the RD on the backup path
!   - Delay/Bandwidth mismatch making backup path cost too high

! Step 3: Tune metrics to bring the path within Feasibility
interface Gi0/2
 delay 10
! Lower delay reduces the metric

! Step 4: Or, adjust offset-list if used
router eigrp 1234
 no offset-list MATCH_ROUTE in 1000 Gi0/2

! Step 5: Verify FS is now eligible
show ip eigrp topology
! Backup path should show as feasible successor

6 Summarization Blackhole (Null0 Missing) Critical

Syslog Message

%DUAL-5-NBRCHANGE: EIGRP-IPv4 1234: Neighbor 10.1.12.2 (Gi0/1) is up: new adjacency
%IP-4-DUPADDR: Duplicate address 10.1.10.1 on GigabitEthernet0/1, sourced by 0000.0c07.ac01
%ROUTING-EIGRP-6-SUMMARY: EIGRP summary route removed for 10.1.0.0/16

Root Cause

A summary route was removed (manually or by configuration reload), and no Null0 discard route exists for the summary prefix. Traffic for specific subnets that are no longer available gets routed to the summary's next-hop — creating a routing blackhole because no more-specific route exists.

Business Impact

Traffic to unreachable subnets is silently dropped or looped
Blackhole persists until the summary is re-added or a specific route appears
Silent failure — no ICMP unreachable by default
Difficult to troubleshoot without checking the Null0 route

Production Fix

! Step 1: Verify the summary route is installed
show ip route | include Null0
show ip eigrp topology | include summary

! Step 2: Re-add the summary route
interface GigabitEthernet0/0
 ip summary-address eigrp 1234 10.16.0.0 255.255.240.0
! This automatically installs a Null0 route with AD 5

! Step 3: Verify Null0 discard route is present
show ip route | include Null0
! Should show: D 10.16.0.0/20 is a summary, Null0

! Step 4: Verify no blackhole remains
show ip route 10.16.0.0
! Should show summary route with Null0 as next hop