OSPF Adjacency Flapping Due to Unstable Interface
Presenting Symptom
OSPF neighbor adjacency repeatedly transitions between FULL and DOWN states, causing routing table instability.
Network Context
A small branch office with two Cisco 4321 routers (R1 and R2) connected via a FastEthernet0/0 link. Both routers run IOS 15.9 and OSPFv2 in area 0. The link experiences intermittent packet loss due to a faulty cable, leading to OSPF hello packet drops.
Diagnostic Steps
Check OSPF neighbor state
show ip ospf neighborNeighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 FULL/DROTHER 00:00:35 10.1.1.2 FastEthernet0/0 192.168.1.2 1 DOWN/DROTHER - 10.1.1.2 FastEthernet0/0
If the neighbor state alternates between FULL and DOWN, it indicates adjacency flapping. A stable adjacency should remain FULL.
Check interface status and errors
show interfaces FastEthernet0/0FastEthernet0/0 is up, line protocol is up
Hardware is Fast Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s, 100BaseTX/FX
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
12345 packets input, 1234567 bytes
Received 123 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
12345 packets output, 1234567 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped outLook for input errors, CRC errors, or interface resets. High error counts suggest physical layer issues like faulty cables or duplex mismatch.
Check OSPF event logs
show logging | include OSPF*Mar 1 00:01:23.456: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.1.2 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Dead timer expired *Mar 1 00:01:28.789: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.1.2 on FastEthernet0/0 from DOWN to FULL, Neighbor Up
Repeated adjacency changes with 'Dead timer expired' indicate hello packets are not being received consistently, likely due to link instability.
Check OSPF interface parameters
show ip ospf interface FastEthernet0/0FastEthernet0/0 is up, line protocol is up
Internet Address 10.1.1.1/24, Area 0
Process ID 1, Router ID 192.168.1.1, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 192.168.1.2, Interface address 10.1.1.2
Backup Designated router (ID) 192.168.1.1, Interface address 10.1.1.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 192.168.1.2 (Designated Router)
Suppress hello for 0 neighbor(s)Verify that Hello and Dead timers match on both sides. Mismatched timers can cause flapping. Also check that the interface is not administratively down or in a passive state.
Root Cause
A faulty Ethernet cable causes intermittent packet loss, leading to OSPF hello packets being dropped. When the dead timer expires, the neighbor is declared down, but once the link recovers, the adjacency re-establishes, resulting in continuous flapping.
Resolution
Verification
Run 'show ip ospf neighbor' and confirm the neighbor state is FULL and stable. Also run 'show interfaces FastEthernet0/0' and verify no input errors or interface resets. Expected output: R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 FULL/DR 00:00:35 10.1.1.2 FastEthernet0/0 R1# show interfaces FastEthernet0/0 | include errors 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored 0 output errors, 0 collisions, 0 interface resets
Prevention
1. Use high-quality, certified Ethernet cables and ensure proper termination. 2. Implement interface error monitoring and alerting to detect physical layer issues early. 3. Configure OSPF with appropriate dead timer intervals to tolerate brief packet loss (e.g., increase dead interval to 120 seconds on lossy links).
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify the cause of OSPF adjacency flapping. The exam tests your ability to interpret 'show ip ospf neighbor' and 'show interfaces' output, and to correlate physical layer issues with OSPF behavior. Key fact: OSPF dead timer expiration due to hello packet loss is a common cause of flapping.
Exam Tips
Remember that OSPF adjacency flapping is often due to Layer 1/2 issues (cable, duplex, speed) rather than OSPF misconfiguration.
In the exam, if you see 'Dead timer expired' in logs, immediately suspect hello packet loss; check interface errors and duplex mismatch.
Know that 'show ip ospf neighbor' output showing alternating FULL and DOWN states is the classic symptom of flapping.
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 ospf interface
Displays OSPF interface parameters and status, used to verify OSPF configuration and troubleshoot neighbor relationships.
show ip ospf neighbor
Displays OSPF neighbor information to verify adjacency formation and troubleshoot OSPF neighbor relationships.
show logging
Displays the state of system logging (syslog) on the device, including buffer contents, logging configuration, and statistics, used for troubleshooting and monitoring system events.
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