Scenario 1 - OSPF Full Neighborhip is Not Coming Up Between Routers
Possible Causes:
Hello and Dead timer or Area ID, Authentication password/type/key, or Area type are mismatched between routers.
Trying to build OSPF neighborship on secondary address.
OSPF not enabled on the correct interface, or network command is wrong.
Network type is NBMA and no neighbor map is configured with the broadcast option.
High CPU or OSPF packet is dropped by the interface due to queuing, high rate, or hardware issue from interface to CPU path.
Mismatch in Subnet mask configuration.
"Passive interface <>" is configured under "router ospf" for the interface.
Mismatch in Network type configuration.
Router is configured with ip ospf priority 0 on the router.
Neighborship is getting built over a virtual link on a stub area.
Solution:
Make sure the hello-dead timers, area ID, area type, and authentication type/password are correct and the same on both routers.
Ensure MTU is the same on both routers.
Ensure the neighbor command is configured on the remote router with the broadcast option.
Make sure OSPF neighborship is built on the primary address.
Ensure access-list/control plane is not dropping the OSPF packets and is allowing OSPF multicast and interface IP address communication.
Ensure the subnet mask is the same on both routers.
Verify that no corrupted OSPF packets are received.
Ensure passive interface is not configured under "router ospf".
Verify that a virtual link is not configured over a stub area.
Scenario 2 - Route is Flapping Across the Network in OSPF
Possible Causes:
Link flapping on the router.
Solution:
Use summarization on the router to reduce the impact of the flapping route.
Use ip event dampening on the interface.
Scenario 3 - OSPF Route is Not Learnt in OSPF Database
Possible Causes:
Originating router not generating LSA.
Routes are redistributed in stub areas.
LSA filter-list is configured on the ABR.
Solution:
Make sure the OSPF adjacency is up.
Ensure redistribution is used with the subnet keyword.
Do not redistribute external routes in a stub area.
Make sure LSA filters are not configured.
Scenario 4 - OSPF Route is Not Installed in the Routing Table
Possible Causes:
Distribute list is configured.
Network type mismatch between routers.
Adv-router not reachable message.
One side is numbered and the other unnumbered (O, O IA, O E1, O E2).
IP addresses are flipped, dual serial (O, O IA, O E1, O E2).
Forwarding address is not known or is known via external/static (O E1, O E2) — routes summed and redistributed.
Backbone area became discontiguous (O, OIA, OE1, OE2).
OSPF is enabled on the secondary but not on the primary.
Solution:
Ensure the distribute-list is configured correctly.
Make sure the forwarding address is reachable for external routes.
Ensure the forwarding address is both reachable and learned via inter-area and intra-area routes.
Ensure network type is the same on both sides.
Ensure there is no backbone partition.
Scenario 5 - OSPF Neighbor is Flapping Between Routers
Possible Causes:
Output and input queue drops/errors/CRC on the interface.
Interface is flapping.
High CPU on the router.
OSPF hello packet is being dropped on the platform between interface and CPU.
Solution:
Ensure no high CPU usage.
Ensure the interface is stable and no packet drops due to errors/CRC/input errors/input queue/output drops.
Ensure no packet drops on the line card and platform.
OSPF Troubleshooting Commands:
show ip ospf neighbor
show ip ospf neighbor detail
show ip ospf
show ip ospf database
show ip ospf interface
show ip ospf database database-summary
show ip ospf virtual-link
show ip ospf database self-originate
show ip ospf database adv-router
show ip ospf statistics
show ip ospf request-list
show ip ospf bad
debug ip ospf adj
debug ip ospf monitor