EIGRP Neighbor Goes Down Without Config Change
Presenting Symptom
An EIGRP neighbor relationship unexpectedly goes down and does not re-establish, even though no configuration changes were made on either router.
Network Context
Two Cisco routers (R1 and R2) are connected via a FastEthernet link in a small branch office. Both run IOS 15.x and are configured with EIGRP AS 100. The network uses a single subnet 192.168.1.0/24. The neighbor had been stable for weeks before suddenly going down.
Diagnostic Steps
Check EIGRP neighbor status
show ip eigrp neighborsNo output or only partial neighbor list (missing the expected neighbor).
If the neighbor is missing, the adjacency is lost. This confirms the problem but does not reveal the cause.
Check EIGRP interface status and parameters
show ip eigrp interfacesInterface FastEthernet0/0 is listed with 'Peers' count 0 or missing entirely.
If the interface is not listed, EIGRP is not enabled on it. If listed but with 0 peers, the interface is up but no neighbor is detected.
Verify Layer 1 and Layer 2 connectivity
show interfaces FastEthernet0/0FastEthernet0/0 is up, line protocol is up (or down).
If line protocol is down, there is a physical or data link issue. If up, the problem is at Layer 3 or above.
Check for interface errors or flapping
show interfaces FastEthernet0/0 | include errors|CRC|collisions|runts|giantsNo errors or minimal errors.
High error counts indicate a faulty cable or interface, which can cause EIGRP hellos to be lost.
Verify EIGRP hello and hold timers
show ip eigrp interfaces detail FastEthernet0/0Hello interval: 5 sec, Hold time: 15 sec
If timers mismatch, adjacency may not form. However, since no config change occurred, mismatch is unlikely.
Check for access lists or route filtering
show ip access-listsNo ACLs applied or ACLs that do not affect EIGRP traffic.
An ACL blocking EIGRP multicast (224.0.0.10) or unicast would prevent hello packets.
Check for passive interface configuration
show running-config | section router eigrppassive-interface default or passive-interface FastEthernet0/0
If the interface is set to passive, EIGRP will not send hellos, causing neighbor loss.
Check for interface shutdown or errdisable
show interfaces FastEthernet0/0 statusFastEthernet0/0 is up (or err-disabled).
If err-disabled, the interface was disabled due to a port security violation or other error.
Root Cause
The interface FastEthernet0/0 on R1 was accidentally placed into errdisable state due to a port security violation (a device with a different MAC address was connected). This caused the interface to go down, dropping all EIGRP hellos and tearing down the neighbor adjacency.
Resolution
Verification
Run 'show ip eigrp neighbors' and verify the neighbor is listed with state 'Init' or 'Pending' then 'Full'. Also run 'show interfaces FastEthernet0/0' to confirm line protocol is up and no errors.
Prevention
1. Use port security with 'restrict' or 'protect' mode instead of 'shutdown' to avoid errdisable. 2. Implement errdisable recovery to automatically re-enable ports after a timeout. 3. Monitor interface status with SNMP or syslog to detect errdisable events early.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario tests troubleshooting EIGRP neighbor adjacency issues. Expect multiple-choice questions asking for the most likely cause when a neighbor goes down without config change, or drag-and-drop steps to diagnose. Key fact: errdisable is a common Layer 2 issue that can break EIGRP adjacency.
Exam Tips
Remember that errdisable is a Layer 2 issue that can cause Layer 3 problems like EIGRP neighbor loss.
Know the 'show interfaces status' command to quickly see if an interface is err-disabled.
Be familiar with errdisable recovery commands and port security violation modes.
Commands Used in This Scenario
show interfaces
Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.
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 port-security
Displays the port security configuration and status on switch interfaces, used to verify and troubleshoot port security settings.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
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