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In the above diagram, Router 1's router-id is 1.1.1.1 and Router 2's router-id is 2.2.2.2.
Note: The router-id will be selected based on the highest IP address on the loopback interface IP. If the loopback interface is not present, the highest physical active interface IP will be preferred as the OSPF router id.
In OSPF, the router-id should be unique on both devices to form the OSPF neighbourship.
# get router info ospf neighbor
OSPF process 0, VRF 0:
Neighbor ID Pri State Dead Time Address Interface
10.0.5.231 1 Full/DR 00:00:38 10.5.21.231 port1
neutron-esx34 # OSPF: LSA[Refresh]: timer expired
OSPF: IFSM[port1:10.5.27.232]: Hello timer expire
OSPF: SEND[Hello]: To 224.0.0.5 via port1:10.5.27.232, length 48 <--- sending hello out via port1:10.5.27.232
OSPF: -----------------------------------------------------
OSPF: Header
OSPF: Version 2
OSPF: Type 1 (Hello)
OSPF: Packet Len 48
OSPF: Router ID 1.1.1.1 <--- the sending router id is 1.1.1.1
OSPF: Area ID 0.0.0.0
OSPF: Checksum 0xd9c6
OSPF: AuType 0
OSPF: Hello
OSPF: NetworkMask 255.255.240.0
OSPF: HelloInterval 10
OSPF: Options 0x2 (*|-|-|-|-|-|E|-)
OSPF: RtrPriority 0
OSPF: RtrDeadInterval 40
OSPF: DRouter 10.5.21.231
OSPF: BDRouter 0.0.0.0
OSPF: # Neighbors 1
OSPF: Neighbor 10.0.5.231
OSPF: -----------------------------------------------------
OSPF: RECV[Hello]: From 1.1.1.1 via port1:10.5.27.232 (10.5.28.246 -> 224.0.0.5) <--- receiving hello via port1:10.5.27.232
OSPF: -----------------------------------------------------
OSPF: Header
OSPF: Version 2
OSPF: Type 1 (Hello)
OSPF: Packet Len 48
OSPF: Router ID 1.1.1.1 <--- while receiving router ID 1.1.1.1
OSPF: Area ID 0.0.0.0
OSPF: Checksum 0xd9c6
OSPF: AuType 0
OSPF: Hello
OSPF: NetworkMask 255.255.240.0
OSPF: HelloInterval 10
OSPF: Options 0x2 (*|-|-|-|-|-|E|-)
OSPF: RtrPriority 0
OSPF: RtrDeadInterval 40
OSPF: DRouter 10.5.21.231
OSPF: BDRouter 0.0.0.0
OSPF: # Neighbors 1
OSPF: Neighbor 10.0.5.231
OSPF: -----------------------------------------------------
OSPF: RECV[Hello]: duplicate router-id 1.1.1.1 detected on port1:10.5.27.232 <--- duplicate router-ID
OSPF: LSA[MaxAge]: Maxage walker finished (0.000000 sec)
After correcting router2's router id as 2.2.2.2, the OSPF neighbourship goes up successfully.
# get router info ospf neighborship is up
OSPF process 0, VRF 0:
Neighbor ID Pri State Dead Time Address Interface
10.0.5.231 1 Full/DR 00:00:35 10.5.21.231 port1
1.1.1.1 0 Full/DROther 00:00:35 10.5.28.246 port1==============>R1 router id 1.1.1.1
# get router info ospf neighbor
OSPF process 0, VRF 0:
Neighbor ID Pri State Dead Time Address Interface
10.0.5.231 1 Full/DR 00:00:38 10.5.21.231 port1
2.2.2.2 1 Full/Backup 00:00:33 10.5.27.232 port1==============>R2 router id 2.2.2.2
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