Part 3 of 5 · Troubleshooting Vault

EIGRP Module D · Enterprise Routing Master Series

Troubleshooting Vault & Production Error Reference

Six real-world EIGRP summarization, stub routing, and filtering 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 for edge and boundary deployments.

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 Null0 Missing After Summarization (Blackhole) Critical

Syslog Message

%ROUTING-EIGRP-6-SUMMARY: EIGRP summary route removed for 10.1.0.0/16
%IP-4-DUPADDR: Duplicate address 10.1.10.1 on GigabitEthernet0/1

Root Cause

The summary route was removed (either manually via configuration change or by a reload that did not restore it), and no Null0 discard route exists. Traffic for specific subnets that fall within the summary range but are no longer advertised gets routed to the summary's next-hop — creating a blackhole or loop.

Business Impact

Traffic to specific subnets is silently dropped or looped
Users cannot reach affected services
Blackhole persists until the summary is re-added or a specific route appears
Difficult to diagnose because no explicit error is logged

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 GigabitEthernet0/1
 ip summary-address eigrp 1234 10.16.0.0 255.255.240.0
 ! Automatically installs a Null0 route with AD 5

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

! Step 4: Test reachability
show ip route 10.16.5.0
! Should show summary route with Null0 as next hop
! OR specific route if the subnet exists

2 Stub Router Causing Adjacency Drop 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 an EIGRP stub but its neighbors still expect it to forward queries. When the neighbor sends a query that the stub cannot answer, the neighbor tears down the adjacency. This commonly happens when stub is deployed on a transit router or during an incomplete rollout where only one side is configured.

Business Impact

Adjacency drops unexpectedly after stub configuration change
All routes via the affected neighbor are removed
Query storms from neighbors trying to find alternatives
Requires careful design coordination, not a casual config change

Production Fix

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

! Step 2: Confirm the design intent
! Is this router supposed to be a stub (edge)?
! Or is it a transit router that should forward queries?

! Step 3: If it should NOT be a stub (transit):
router eigrp 1234
 no eigrp stub

! Step 4: If it should be a stub, correct the options
router eigrp 1234
 eigrp stub connected summary

! Step 5: Verify adjacencies return
show ip eigrp neighbors
show ip eigrp neighbors detail | include stub

3 Distribute-List Blocking Essential Routes Critical

Syslog Message

! No explicit syslog — routes simply go missing
%DUAL-5-NBRCHANGE: EIGRP-IPv4 1234: Neighbor 10.1.12.2 (Gi0/1) is up: new adjacency
! But no routes are learned from that neighbor

Root Cause

A distribute-list was applied inbound to filter routes, but the prefix-list or ACL is too restrictive and blocks essential routes. This often happens when the ACL was intended to filter one specific subnet but a typo in the wildcard mask causes it to match unintended prefixes.

Business Impact

Routes silently disappear from the routing table
Connectivity to affected subnets is lost
Silent failure — no warning in logs
Extended troubleshooting time due to the lack of error messages

Production Fix

! Step 1: Verify which distribute-list is applied
show ip protocols | include filter
show run | section distribute-list
show ip prefix-list

! Step 2: Test the ACL or prefix-list logic
show ip prefix-list FILTER
show ip prefix-list FILTER detail

! Step 3: Temporarily remove the filter to confirm it is the cause
router eigrp 1234
 no distribute-list prefix FILTER in GigabitEthernet0/1

! Step 4: Verify routes appear
show ip route eigrp

! Step 5: Correct the filter and reapply
ip prefix-list FILTER permit 10.1.10.0/24
ip prefix-list FILTER permit 10.1.20.0/24
router eigrp 1234
 distribute-list prefix FILTER in GigabitEthernet0/1

4 Redistribution Loop from Missing Route Tag Critical

Syslog Message

! No explicit syslog — route instability observed
%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
! Repeated neighbor flaps + route churn across redistribution boundary

Root Cause

Routes were redistributed from OSPF into EIGRP without a route tag. EIGRP then redistributed the same routes back into OSPF (via a second redistribution point). OSPF accepted them as new routes, which EIGRP then learned again as external — creating a redistribution loop. The result is route instability and neighbor flapping.

Business Impact

Routes continuously flip between internal and external
Neighbor adjacencies flap repeatedly
CPU spikes from route recalculation
Extended outage if loop is not broken quickly

Production Fix

! Step 1: Identify redistributed routes without tags
show ip route eigrp | include EX
show ip route 172.16.0.0 detail | include Tag

! Step 2: Apply route tags on the OSPF-to-EIGRP redistribution
route-map OSPF_TO_EIGRP permit 10
 match ip address prefix-list REDIST
 set tag 100
 set metric 100000 100 255 1 1500

! Step 3: Deny tagged routes on the reverse redistribution (EIGRP-to-OSPF)
route-map EIGRP_TO_OSPF deny 5
 match tag 100
route-map EIGRP_TO_OSPF permit 10
 match ip address prefix-list REDIST

! Step 4: Apply consistently on all redistribution points
router ospf 1
 redistribute eigrp 1234 subnets route-map EIGRP_TO_OSPF
router eigrp 1234
 redistribute ospf 1 route-map OSPF_TO_EIGRP

! Step 5: Verify loop is broken
show ip route 172.16.0.0 detail
! Tag 100 should be present; route should not flip

5 Summarization on Wrong Interface → Suboptimal Routing Warning

Syslog Message

! No syslog — observed via traffic analysis
! Remote routers are routing traffic back to the summarization point
! for specific subnets that should have direct paths

Root Cause

Summarization was configured on one interface but not on all relevant interfaces of the summarizing router. Specific subnets leaked to some neighbors while being summarized to others. When a specific subnet fails, the summary still routes traffic to the router that lost the route — creating a suboptimal path or loop.

Business Impact

Traffic takes a longer path than necessary (suboptimal routing)
Latency spikes for specific destinations
Potential for routing loops between summary and specific routes
Difficult to diagnose because each router sees a different view

Production Fix

! Step 1: Verify summarization is applied on ALL relevant interfaces
show run interface | include summary-address
show ip protocols | include summary

! Step 2: Check which interfaces have summarization and which do not
show ip route | include Null0

! Step 3: Apply summarization on every interface facing the summarized domain
interface GigabitEthernet0/1
 ip summary-address eigrp 1234 10.16.0.0 255.255.240.0
interface GigabitEthernet0/2
 ip summary-address eigrp 1234 10.16.0.0 255.255.240.0

! Step 4: Verify consistent advertisement
show ip route | include Null0
! Should show Null0 for the summary route

! Step 5: Confirm remote routers see only summary
! (on remote router)
show ip route | include 10.16
! Should show summary route with the local router as next-hop

6 Prefix-List Typo → Unintended Route Filtering Warning

Syslog Message

! No syslog — silent route filtering
! Essential routes are missing from the routing table
! No NBRCHANGE or DUAL events visible

Root Cause

A prefix-list was created with a typo in the network address or prefix length. For example, permit 10.1.10.0/24 was written as permit 10.1.10.0/25, or a subnet mask was written as 255.255.255.0 instead of 10.1.10.0/24. The result is that the intended routes are not matched and therefore not permitted — they are silently denied.

Business Impact

Routes silently disappear from the topology table
No error is logged — very hard to detect
Users lose connectivity to specific destinations
Difficult to diagnose without careful prefix-list review

Production Fix

! Step 1: Inspect the prefix-list in detail
show ip prefix-list FILTER
show ip prefix-list FILTER detail

! Step 2: Verify the intended prefixes
! Confirm the network address and prefix length are correct
! Example correct entry:
ip prefix-list FILTER permit 10.1.10.0/24

! Step 3: Test with a specific route
! Temporarily disable the filter and see if the route appears
router eigrp 1234
 no distribute-list prefix FILTER in GigabitEthernet0/1

show ip route 10.1.10.0
! If the route appears now, the filter was the cause

! Step 4: Fix the typo and reapply
ip prefix-list FILTER permit 10.1.10.0/24
ip prefix-list FILTER permit 10.1.20.0/24
router eigrp 1234
 distribute-list prefix FILTER in GigabitEthernet0/1

! Step 5: Verify the route is now accepted
show ip route 10.1.10.0
show ip eigrp topology | include 10.1.10