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OSPFCCNA 200-301

OSPF Network Type Mismatch on Ethernet Segment

Presenting Symptom

Two directly connected routers on the same Ethernet segment fail to form an OSPF neighbor adjacency.

Network Context

Two Cisco routers (R1 and R2) are connected via a single Ethernet switch in a small branch office. Both routers run IOS 15.x and are configured with OSPF in area 0. The link between them is a standard Ethernet segment (not a point-to-point link). The OSPF process is enabled on the interface, but the neighbor state remains stuck in INIT or EXSTART.

Diagnostic Steps

1

Check OSPF neighbor status

show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.1.2      1   INIT/ -        00:00:33    192.168.1.2     GigabitEthernet0/0

If the neighbor state is INIT or EXSTART/EXCHANGE and not FULL, there is a problem with OSPF parameter mismatch. The state INIT indicates that a hello packet was received but parameters do not match.

2

Verify OSPF interface configuration on both routers

show ip ospf interface GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
  Internet Address 192.168.1.1/24, Area 0
  Process ID 1, Router ID 192.168.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1
  ...

Look at the 'Network Type' field. On an Ethernet segment, the default network type is BROADCAST. If one router shows a different network type (e.g., POINT-TO-POINT), that is the root cause of the mismatch.

3

Check OSPF hello and dead timers

show ip ospf interface GigabitEthernet0/0 | include Hello|Dead
  Hello due in 00:00:03
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Verify that Hello and Dead intervals match on both routers. Mismatched timers can also cause adjacency issues, but in this scenario the network type mismatch is the primary cause.

4

Confirm network type configuration on both routers

show running-config interface GigabitEthernet0/0
interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 ip ospf network point-to-point

If one router has 'ip ospf network point-to-point' configured and the other does not (defaults to broadcast), the network types are mismatched. This prevents adjacency formation because OSPF uses different hello and dead timers for different network types.

Root Cause

The OSPF network type is mismatched between the two routers on the same Ethernet segment. One router is configured with 'ip ospf network point-to-point' (or another non-default type) while the other uses the default broadcast network type. OSPF requires matching network types on the same link to form a neighbor adjacency.

Resolution

On the router with the non-default network type, remove the 'ip ospf network point-to-point' command to revert to the default broadcast type, or configure the same network type on both routers. For broadcast: R1(config)# interface GigabitEthernet0/0 R1(config-if)# no ip ospf network point-to-point Alternatively, configure both as point-to-point if the link is physically point-to-point: R1(config)# interface GigabitEthernet0/0 R1(config-if)# ip ospf network point-to-point R2(config)# interface GigabitEthernet0/0 R2(config-if)# ip ospf network point-to-point

Verification

Run 'show ip ospf neighbor' on both routers. Expected output: Neighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 FULL/DR 00:00:35 192.168.1.2 GigabitEthernet0/0 The neighbor state should be FULL, indicating adjacency is established.

Prevention

1. Standardize OSPF network type configuration across all routers in the same broadcast segment. 2. Use the default broadcast network type on Ethernet segments unless there is a specific reason to change it (e.g., point-to-point links). 3. Document and verify OSPF interface parameters during initial deployment.

CCNA Exam Relevance

On the CCNA 200-301 exam, OSPF network type mismatch appears in troubleshooting scenarios, often as a multiple-choice question or a drag-and-drop where you must identify the cause and solution. The exam tests your understanding that OSPF network types must match on a link, and that mismatched types prevent neighbor adjacency. Key fact: The default OSPF network type on Ethernet is broadcast; point-to-point is used on serial or point-to-point Ethernet links.

Exam Tips

1.

Remember that OSPF network type mismatch is a common cause of adjacency failure; always check 'show ip ospf interface' for network type.

2.

On the exam, if you see a scenario with two routers on a switch and they are stuck in INIT state, suspect network type mismatch or timer mismatch.

3.

Know that 'ip ospf network point-to-point' disables DR/BDR election and uses a 10-second hello timer (same as broadcast), but the network type must match.

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.

Test Your CCNA Knowledge

Practice with scenario-based questions to prepare for the CCNA 200-301 exam.

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