OSPF Neighbor States
- Down: No Hello packets received.
- Attempt (Only in NBMA networks): waiting for Hello reply after actively sending.
- Init: Hello received, but own Router ID not seen by neighbor yet.
- 2-Way: Hello exchanged, bidirectional communication confirmed.
- ExStart/Exchange: Master/slave negotiation, start of database exchange.
- Loading/Full: LSAs exchanged; Full means neighbor is fully established.
The Down state means no Hello packets are received. This state does not appear in show ip ospf neighbor output.
1. Router ID conflict
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R1#
*May 28 22:37:20.707: %OSPF-4-DUPRTRIDNBR: OSPF detected duplicate router-id 1.1.1.1 from 12.1.1.2 on interface GigabitEthernet0/0
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R2#
*May 28 22:37:21.379: %OSPF-4-DUPRTRIDNBR: OSPF detected duplicate router-id 1.1.1.1 from 12.1.1.1 on interface GigabitEthernet0/0
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2. Area ID mismatch
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R1#debug ip ospf adj
May 28 22:33:36.823: OSPF-110 ADJ Gi0/0: Rcv pkt from 12.1.1.2, area 0.0.0.0, mismatched area 0.0.0.1 in the header
R1#
May 28 22:33:46.383: OSPF-110 ADJ Gi0/0: Rcv pkt from 12.1.1.2, area 0.0.0.0, mismatched area 0.0.0.1 in the header
R1#
*May 28 22:33:56.119: OSPF-110 ADJ Gi0/0: Rcv pkt from 12.1.1.2, area 0.0.0.0, mismatched area 0.0.0.1 in the header
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May 28 22:33:34.491: %OSPF-4-ERRRCV: Received invalid packet: mismatched area ID, from backbone area must be virtual-link but not found from 12.1.1.1, GigabitEthernet0/0
R2#
May 28 22:33:43.527: %OSPF-4-ERRRCV: Received invalid packet: mismatched area ID, from backbone area must be virtual-link but not found from 12.1.1.1, GigabitEthernet0/0
R2#
*May 28 22:33:52.651: %OSPF-4-ERRRCV: Received invalid packet: mismatched area ID, from backbone area must be virtual-link but not found from 12.1.1.1, GigabitEthernet0/0
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3. OSPF authentication mismatch
- Different authentication types:
R1 is configured with OSPF authentication, whereas R2 is not.
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R1#sh run int g0/0 | in ospf
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 chen
R1#sh ip os int g0/0
Message digest authentication enabled
Youngest key id is 1
R1#sh ip os nei
R1#
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R1#debug ip os adj
OSPF adjacency debugging is on
R1#
May 28 22:54:20.947: OSPF-110 ADJ Gi0/0: Send with youngest Key 1
R1#
May 28 22:54:29.199: OSPF-110 ADJ Gi0/0: Rcv pkt from 12.1.1.2 : Mismatched Authentication type. Input packet specified type 0, we use type 2
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R2#sh run int g0/0 | i ospf
R2#sh ip os ne
R2#
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- Different authentication keys:
R1 and R2 have the same authentication enabled, but with different keys.
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R2#sh run int g0/0 | in ospf
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 cai
R2#sh ip os int g0/0
Message digest authentication enabled
Youngest key id is 1
R2#sh ip os nei
R2#
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May 28 23:02:22.319: OSPF-110 ADJ Gi0/0: Rcv pkt from 12.1.1.2 : Mismatched Authentication Key - Message Digest Key 1
R1#
May 28 23:02:23.783: OSPF-110 ADJ Gi0/0: Send with youngest Key 1
R1#
*May 28 23:02:31.815: OSPF-110 ADJ Gi0/0: Rcv pkt from 12.1.1.2 : Mismatched Authentication Key - Message Digest Key 1
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4. Subnet Mask Mismatch
R1: 12.1.1.1/24
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R1#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.1/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 12.1.1.1
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
R1#sh ip os nei
R1#
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R2: 12.1.1.2/25
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R2#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.2/25, Area 0, Attached via Network Statement
Process ID 110, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 2.2.2.2, Interface address 12.1.1.2
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
R2#sh ip os nei
R2#
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Generally, OSPF neighbors must be in the same IP subnet to form adjacency.
Even if devices can ping each other, different subnet masks (e.g., /24 vs /25) cause OSPF to treat the interfaces as not on the same network. Hello packets are ignored, and neighbors are not formed.
Exception: point-to-point/multipoint
On point-to-point and point-to-multipoint network types, OSPF does not require the interfaces to have matching subnet masks.
R1: 12.1.1.1/24, Network Type POINTTOPOINT
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R1#sh ip ospf interface g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.1/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 1.1.1.1, Network Type POINTTOPOINT, Cost: 1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 2.2.2.2
R1#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 00:00:33 12.1.1.2 GigabitEthernet0/0
R1#
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R2: 12.1.1.2/25, Network Type POINTTOPOINT
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R2#sh ip os interface g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.2/25, Area 0, Attached via Network Statement
Process ID 110, Router ID 2.2.2.2, Network Type POINTTOPOINT, Cost: 1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 1.1.1.1
R2#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 0 FULL/ - 00:00:36 12.1.1.1 GigabitEthernet0/0
R2#
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R1#sh ip os int g0/0
Internet Address 12.1.1.1/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 1.1.1.1, Network Type POINTTOMULTIPOINT, Cost: 1
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
R1#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 00:01:58 12.1.1.2 GigabitEthernet0/0
R1#
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R2#sh ip os interface g0/0
Internet Address 12.1.1.2/25, Area 0, Attached via Network Statement
Process ID 110, Router ID 2.2.2.2, Network Type POINTTOMULTIPOINT, Cost: 1
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
R2#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 0 FULL/ - 00:01:53 12.1.1.1 GigabitEthernet0/0
R2#
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5. OSPF Network Type Mismatch
R1: Network Type POINTTOPOINT
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R1#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.1/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 1.1.1.1, Network Type POINTTOPOINT, Cost: 1
Transmit Delay is 1 sec, State POINTTOPOINT
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
R1#sh ip os nei
R1#
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R2: Network Type POINTTOMULTIPOINT
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R2#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.2/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 2.2.2.2, Network Type POINTTOMULTIPOINT, Cost: 1
Transmit Delay is 1 sec, State POINTTOMULTIPOINT
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
R2#sh ip os nei
R2#
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Exception: broadcast & point-to-point
By default, OSPF adjacency fails between point-to-point and point-to-multipoint, but can form between point-to-point and broadcast because they share the same default Hello and Dead intervals.
R1: Network Type POINTTOPOINT, Hello 10, Dead 40
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R1#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.1/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 1.1.1.1, Network Type POINTTOPOINT, Cost: 1
Transmit Delay is 1 sec, State POINTTOPOINT
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
R1#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 00:00:39 12.1.1.2 GigabitEtherne t0/0
R1#
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R2: Network Type BROADCAST, Hello 10, Dead 40
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R2#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.2/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 2.2.2.2, Interface address 12.1.1.2
Backup Designated router (ID) 1.1.1.1, Interface address 12.1.1.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
R2#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/BDR 00:00:36 12.1.1.1 GigabitEtherne t0/0
R2#
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Exception: manually adjust the Hello/Dead intervals
R1:Network Type POINTTOPOINT; using default Hello/Dead intervals
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R1#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.1/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 1.1.1.1, Network Type POINTTOPOINT, Cost: 1
Transmit Delay is 1 sec, State POINTTOPOINT
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
R1#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 00:00:35 12.1.1.2 GigabitEthernet0/0
R1#
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R2: Network type POINTTOMULTIPOINT; Hello interval changed to 10
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R2#sh run int g0/0 | in ip ospf
ip ospf network point-to-multipoint
ip ospf hello-interval 10
R2#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.2/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 2.2.2.2, Network Type POINTTOMULTIPOINT, Cost: 1
Transmit Delay is 1 sec, State POINTTOMULTIPOINT
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
R2#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 0 FULL/ - 00:00:37 12.1.1.1 GigabitEthernet0/0
R2#
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OSPF adjacencies can form between different network types if Hello and Dead intervals match, but this may lead to LSA flooding issues.
6. Hello/Dead interval mismatch
When hello-interval is modified, dead-interval automatically adjusts to 4 times the hello value.
But when dead-interval is modified, hello-interval does not change.
R1: Dead interval changed to 50
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R1#sh run int g0/0 | in ip ospf
ip ospf dead-interval 50
R1#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.1/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 12.1.1.1
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 50, Wait 50, Retransmit 5
R1#sh ip os nei
R1#
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R2#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.2/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 2.2.2.2, Interface address 12.1.1.2
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
R2#sh ip os nei
R2#
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Exception: hello-interval < 1 Seconds
R1: hello-interval 333 msec
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R1#sh run int g0/0 | in ospf
ip ospf dead-interval minimal hello-multiplier 3
R1#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.1/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 2.2.2.2, Interface address 12.1.1.2
Backup Designated router (ID) 1.1.1.1, Interface address 12.1.1.1
Timer intervals configured, Hello 333 msec, Dead 1, Wait 1, Retransmit 5
R1#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 FULL/DR 896 msec 12.1.1.2 GigabitEthernet0/0
R1#
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R2: hello-interval 250 msec
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R2#sh run int g0/0 | in ospf
ip ospf dead-interval minimal hello-multiplier 4
R2#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.2/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 2.2.2.2, Interface address 12.1.1.2
Backup Designated router (ID) 1.1.1.1, Interface address 12.1.1.1
Timer intervals configured, Hello 250 msec, Dead 1, Wait 1, Retransmit 5
R2#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/BDR 880 msec 12.1.1.1 GigabitEthernet0/0
R2#
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When hello-interval is configured to less than 1 second, OSPF can form adjacencies even if the Hello/Dead timers are not exactly the same on both sides.
Exception: hello-interval > dead-interval
When the hello-interval is set longer than the dead-interval, OSPF neighbors will form adjacency briefly, but then continuously flap.
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R1#sh run int g0/0 | in ospf
ip ospf dead-interval 8
R1#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.1/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 2.2.2.2, Interface address 12.1.1.2
Backup Designated router (ID) 1.1.1.1, Interface address 12.1.1.1
Flush timer for old DR LSA due in 00:02:46
Timer intervals configured, Hello 10, Dead 8, Wait 8, Retransmit 5
R1#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 EXSTART/DR 00:00:05 12.1.1.2 GigabitEthernet0/0
R1#
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R2#sh run int g0/0 | in ospf
ip ospf dead-interval 8
R2#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.2/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 2.2.2.2, Interface address 12.1.1.2
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 8, Wait 8, Retransmit 5
R2#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/BDR 00:00:07 12.1.1.1 GigabitEthernet0/0
R2#
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May 28 22:19:45.775: %OSPF-5-ADJCHG: Process 110, Nbr 1.1.1.1 on GigabitEthernet0/0 from FULL to DOWN, Neighbor Down: Dead timer expired
R2#
May 28 22:19:51.947: %OSPF-5-ADJCHG: Process 110, Nbr 1.1.1.1 on GigabitEthernet0/0 from LOADING to FULL, Loading Done
R2#
May 28 22:20:04.491: %OSPF-5-ADJCHG: Process 110, Nbr 1.1.1.1 on GigabitEthernet0/0 from FULL to DOWN, Neighbor Down: Dead timer expired
R2#
*May 28 22:20:11.403: %OSPF-5-ADJCHG: Process 110, Nbr 1.1.1.1 on GigabitEthernet0/0 from LOADING to FULL, Loading Done
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7. Stub Area Flag Mismatch
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R2#debug ip ospf hello
OSPF hello debugging is on
R2#
May 28 23:29:43.095: OSPF-110 HELLO Gi0/0: Send hello to 224.0.0.5 area 1 from 12.1.1.2
R2#
May 28 23:29:45.075: OSPF-110 HELLO Gi0/0: Rcv hello from 1.1.1.1 area 1 12.1.1.1
*May 28 23:29:45.075: OSPF-110 HELLO Gi0/0: Hello from 12.1.1.1 with mismatched Stub/Transit area option bit
R2#
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8. No neighbor command on non-broadcast network
On non-broadcast networks like NBMA and Point-to-Multipoint NBMA, OSPF does not send Hello packets automatically and requires manual neighbor configuration with the neighbor command to form adjacencies.
9. Passive interface
A passive interface does not send or process Hello packets, so no OSPF neighbor adjacency is formed on it.
10. Interface is down or cable disconnected
11. OSPF process not running
12. Hello packets blocked by ACL or firewall
13. MTU mismatch (Stuck in ExStart/Exchange state)
R1: MTU 2000
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R1#sh run int g0/0 | in mtu
mtu 2000
R1#sh int g0/0 | in MTU
MTU 2000 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
R1#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 EXCHANGE/BDR 00:00:39 12.1.1.2 GigabitEthernet0/0
R1#
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R2: MTU 1500
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R2#sh int g0/0 | in MTU
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
R2#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 EXSTART/DR 00:00:39 12.1.1.1 GigabitEthernet0/0
R2#
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R1#debug ip ospf adj
OSPF adjacency debugging is on
R1#
May 28 23:38:26.239: OSPF-110 ADJ Gi0/0: Rcv DBD from 2.2.2.2 seq 0x1250 opt 0x52 flag 0x7 len 32 mtu 1500 state EXCHANGE
May 28 23:38:26.239: OSPF-110 ADJ Gi0/0: Nbr 2.2.2.2 has smaller interface MTU
*May 28 23:38:26.239: OSPF-110 ADJ Gi0/0: Send DBD to 2.2.2.2 seq 0x1250 opt 0x52 flag 0x2 len 52
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Exception: ip ospf mtu-ignore
The command ip ospf mtu-ignore allows OSPF to ignore MTU checks, enabling neighbor adjacency even when MTU values are unequal.
It is sufficient to apply this command on the side with the smaller MTU.
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R2#sh run int g0/0 | in ospf
ip ospf mtu-ignore
R2#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DR 00:00:39 12.1.1.1 GigabitEthernet0/0
R2#
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14. MA Network: Priority = 0 (Stuck in 2-Way state)
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R1#sh run int g0/0 | in ospf
ip ospf priority 0
R1#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.1/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DROTHER, Priority 0
No designated router on this network
No backup designated router on this network
Flush timer for old DR LSA due in 00:01:59
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Neighbor Count is 1, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)
R1#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 2WAY/DROTHER 00:00:36 12.1.1.2 GigabitEthernet0/0
R1#
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R2#sh run int g0/0 | in ospf
ip ospf priority 0
R2#sh ip os int g0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 12.1.1.2/24, Area 0, Attached via Network Statement
Process ID 110, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DROTHER, Priority 0
No designated router on this network
No backup designated router on this network
Flush timer for old DR LSA due in 00:01:59
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Neighbor Count is 1, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)
R2#sh ip os nei
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 0 2WAY/DROTHER 00:00:36 12.1.1.1 GigabitEthernet0/0
R2#
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Chen