ospf scenario based questions

ospf scenario based questions

Scenario 1 – 

OSPF full neigborship is not coming up between routers —

1. Hello and Dead timer or Area ID, Authentication password/type/key or Area type are   mismatch between router.
2. Trying to build OSPF neigborship on secondary address.
4.  OSPF not enabled on correct interface or network command is wrong.
5. Network type is NBMA and no neigbor map configured with broadcast option.
6. High CPU  or OSPF packet is dropped by interface due to queuing or high rate or hardware issue from interface to CPU path.
7. Mismatch Subnet mask is configured.
8. “passive interface <> ” is configured under “router ospf”for the interface.
9.  Mismatch Network type is configured.
10. Router is configured with  ip ospf priority 0 on router.
11. Neighborship is getting built over virtual link on sutb area.

Solution of the Above question —

1- Make sure hello-dead/ area id, area type,authentication type/password are correct and same.
2-  Make sure MTU is same on both router.
3- Make sure neigbor command is configured on remote router with broadcast.
4-  Make sure OSPF neigborship build on primary address.
5-  Access-list /control plane is not dropping the packet and allowing OSPF multicast and interface ip address communication.
6 – Subnet mask should be same on the router.
7 –  Make sure no corrupted OSPF packet received.
8 –  Make sure passive interface is not configured under “router ospf”.
9 –  Make sure Virtual -link is not configured over stub area.

Scenario 2 – 

Route is flapping across the network in OSPF

Link flapping on the router.

Solution of above question – 

a) Use summarization on the router to reduce the impact of flapping route.
b) Use” ip event dampening ” on the interface.

Scenario 3- 

OSPF Route is not learnt in OSPF database

1) Originating router not generating LSA.
2) Routes are redistributed in stub areas.
3) LSA filter-list is configured on ABR.

Solution of above question –

1 –  Make sure OSPF adjacency is up.

2- Make sure redistribution is used with subnet keyword as well.

3- Do not redistribute external routes in stub area.
5- Make sure LSA are not filtered.

Scenario 4 – 

OSPF Route is not installed in routing table –

1) Distribute list is configured.
3)Network type mismatch between router.
4) Adv-router not reachable msg.
5)One side is numbered and the other unnumbered (O, O IA, O E1,O E2).
6)ip addresses are flipped, dual serial (O, O IA, O E1, O E2).
7) Forwarding address is not known or is known via external/static (O E1, O E2) – routes sum and redistribute conn?
8) Backbone area became discontigous (O, OIA, OE1, OE2)
9) OSPF is enabled on secondary but not on primary.

Solution of above question –

1- Make sure distribute-list is configured correctly.
2- Make sure forwarding address is reachable for external route.
3- Make sure forwarding address is not even reachable but also should be learned via inter area and intra area.
4- Make sure network type is correct on both sides.
5 – Make sure there is no backbone partition.

Scenario 5 – 

OSPF Neighbor is flapping between router –

1. Output and input queue drops/error/crc on the interface.
2. Interface is flapping .
3. High CPU on the router.
4. OSPF hello packet is dropping on the platform between interface and CPU.

Solution of above question –

1- Make sure  no high CPU.
2- Interface is stable and no packet drops on the interface due to error/crc/input error/input queue/output drops.
3- No packet drops on the line card and platform.

OSPF Troubleshooting commands