EIGRP Passive-Interface Set on Wrong Interface
Presenting Symptom
Routers in the EIGRP domain are not forming neighbor adjacencies, and routes are missing from the routing table.
Network Context
The network is a small branch office with three Cisco routers (R1, R2, R3) running IOS 15.9. They are connected via serial links (R1-R2, R2-R3) and configured with EIGRP AS 100. All interfaces are in the same subnet range. The issue occurs after a recent configuration change where an engineer intended to suppress EIGRP hello packets on a LAN interface but mistakenly applied the passive-interface command to the serial interface connecting to the neighbor.
Diagnostic Steps
Check EIGRP Neighbors
show ip eigrp neighborsNo output or only a single neighbor entry (if any).
If no neighbors are listed, EIGRP is not forming adjacencies. This indicates a problem with hello packet exchange or interface configuration.
Verify EIGRP Interfaces
show ip eigrp interfacesInterface Peers Hold Queue Serial0/0/0 0 0 0/0
If an interface shows 0 peers, it may be passive. Compare with interfaces that should have neighbors.
Check Interface Passive Status
show running-config | section router eigrprouter eigrp 100 network 10.0.0.0 passive-interface Serial0/0/0
The passive-interface command on the serial interface prevents EIGRP hello packets from being sent, thus no neighbor adjacency can form.
Confirm Interface IP and Status
show ip interface briefInterface IP-Address OK? Method Status Protocol Serial0/0/0 10.0.0.1 YES manual up up
The interface is up/up, so the issue is not physical. The problem is the passive-interface configuration.
Root Cause
The passive-interface command was applied to the serial interface (Serial0/0/0) on R1, which prevents EIGRP from sending hello packets out that interface. Without hello packets, R1 cannot discover its neighbor on that link, and no EIGRP adjacency forms.
Resolution
Verification
Run the following commands to confirm neighbor adjacency forms: R1# show ip eigrp neighbors Expected output: H Address Interface Hold Uptime SRTT RTO Q Seq 0 10.0.0.2 Se0/0/0 13 00:00:15 1 100 0 1 R1# show ip route eigrp Expected output includes routes learned via EIGRP.
Prevention
1. Always verify the interface type before applying passive-interface; use it only on LAN interfaces where no EIGRP neighbors exist. 2. Use the 'passive-interface default' command to make all interfaces passive by default, then use 'no passive-interface' on interfaces that need to form adjacencies. 3. After any EIGRP configuration change, verify neighbor adjacencies with 'show ip eigrp neighbors'.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why EIGRP neighbors are not forming. The exam tests knowledge of the passive-interface command's effect on hello packets and neighbor discovery. Candidates must know that passive-interface stops sending hellos but still processes received hellos (though no adjacency forms because hellos are not sent).
Exam Tips
Remember: passive-interface prevents sending EIGRP hellos, so no neighbor adjacency can form on that interface.
The command 'show ip eigrp interfaces' shows which interfaces are passive (Peers column shows 0).
In the exam, look for the passive-interface command under the router eigrp configuration as the root cause of missing neighbors.
Commands Used in This Scenario
show ip eigrp interfaces
Displays detailed information about interfaces on which EIGRP is enabled, including neighbor status, pending routes, and interface statistics, used to verify EIGRP adjacency and interface participation.
show ip eigrp neighbors
Displays the neighbor table for EIGRP, showing all directly connected EIGRP routers and their status, used to verify EIGRP adjacencies and troubleshoot neighbor relationships.
show running-config
Displays the current active configuration in DRAM, showing all non-default settings.
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