Wrong Wildcard Mask in OSPF Network Statement
Presenting Symptom
OSPF neighbors are not forming, and routes are missing from the routing table.
Network Context
A small branch office with two Cisco routers (R1 and R2) connected via a serial link (10.0.0.0/30). Both routers run OSPF with process ID 1. The network engineer configured OSPF on R1 with the command 'network 10.0.0.0 0.0.0.0 area 0' intending to advertise the serial link, but the wildcard mask is incorrect, preventing OSPF from enabling on the interface.
Diagnostic Steps
Check OSPF neighbors
show ip ospf neighborR1# show ip ospf neighbor (no output)
No OSPF neighbors are listed, indicating that OSPF is not forming adjacencies. This could be due to misconfiguration or interface issues.
Verify OSPF interfaces
show ip ospf interfaceR1# show ip ospf interface
Serial0/0/0 is up, line protocol is up
Internet Address 10.0.0.1/30, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64
Transmit Delay is 1 sec, State POINT_TO_POINT
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:04
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1/1, flood queue length 0
Next 0x0(0)/0x0(0)/0x0(0)
Last flood scan length is 0, maximum is 0
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 0, Adjacent neighbor count is 0
Suppressed hello packets 0
No Hellos (Passive interface)The interface shows 'No Hellos (Passive interface)', indicating that OSPF is not sending Hellos on this interface. This is abnormal for a non-passive interface. The likely cause is that the network statement does not match the interface IP address due to an incorrect wildcard mask.
Examine OSPF configuration
show running-config | section router ospfR1# show running-config | section router ospf router ospf 1 network 10.0.0.0 0.0.0.0 area 0
The network statement uses wildcard mask 0.0.0.0, which matches only the exact IP address 10.0.0.0. However, the interface IP is 10.0.0.1/30, so the network statement does not match. OSPF will not enable on the interface.
Check interface IP address
show ip interface briefR1# show ip interface brief Interface IP-Address OK? Method Status Protocol Serial0/0/0 10.0.0.1 YES manual up up
The interface IP is 10.0.0.1, which is not covered by the network statement with wildcard 0.0.0.0. The correct wildcard mask should be 0.0.0.3 to match the /30 subnet.
Root Cause
The OSPF network statement on R1 uses a wildcard mask of 0.0.0.0, which matches only the exact IP address 10.0.0.0. However, the interface Serial0/0/0 has IP address 10.0.0.1/30, so OSPF does not enable on that interface, preventing neighbor formation and route exchange.
Resolution
Verification
Verify OSPF neighbors and interfaces. R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 0 FULL/ - 00:00:38 10.0.0.2 Serial0/0/0 R1# show ip ospf interface serial0/0/0 Serial0/0/0 is up, line protocol is up Internet Address 10.0.0.1/30, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:02 Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 2.2.2.2 Suppressed hello packets 0 Now the neighbor is FULL and Hellos are being sent.
Prevention
1. Always use the correct wildcard mask that matches the subnet mask of the interface. For a /30 subnet, use 0.0.0.3. 2. Use the 'network' command with the exact subnet and wildcard mask, or consider using 'ip ospf' interface configuration to avoid wildcard mask errors. 3. Verify OSPF interface status after configuration to ensure OSPF is enabled on the intended interfaces.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where OSPF neighbors fail to form. The exam tests the understanding of OSPF network statements and wildcard masks. Candidates must know that the wildcard mask is the inverse of the subnet mask and that a mismatch prevents OSPF from enabling on the interface. Expect multiple-choice or drag-and-drop questions asking to identify the misconfiguration or correct the network statement.
Exam Tips
Remember that the wildcard mask is the inverse of the subnet mask (e.g., /30 = 255.255.255.252, wildcard = 0.0.0.3).
If OSPF neighbors are not forming, always check 'show ip ospf interface' for 'No Hellos' or 'Passive interface' to identify if OSPF is enabled on the interface.
The 'network' command uses wildcard mask, not subnet mask. A common mistake is using the subnet mask instead of wildcard mask.
Commands Used in This Scenario
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 running-config
Displays the current active configuration in DRAM, showing all non-default settings.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
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