OSPF stuck in INIT (one way hello) 
 Multicast is broken or layer 2 problem.
Access-list is blocking ospf multicast address.
OSPF hello packet getting NAT translated.
 Layer 2 is broken. 
   
OSPF stuck in 2-WAY
 Normal on ethernet broadcast.
 Layer 2 is broken.
All routers are configured with priority 0 so
there will not be any election. 

OSPF stuck in EXSTART/EXCHANGE
 MTU mismatch between neighbor 
 Duplicate router-ID between routers.
 Packet loss can also cause to stuck. 
 Access-list is blocking unicast
communication between router.

OSPF stuck in LOADING
 Neigbor is sending bad packet or corrupt packet due to memory.
 LS request packet is not accepting by
neighbor and ignoring. 

Debug command :
debug ip ospf adj

Solution:
a) Make sure hello-dead/ area id, area
type,authentication type/password are correct and same.
b) Make sure MTU is same on both router.
c) Make sure neigbor command is configured on
remote router with broadcast.
d) Make sure OSPF neigborship build on primary
address.
e) Access-list /control plane is not dropping
the packet and allowing OSPF multicast and interface ip address communication.
f) Subnet mask should be same on the router.
g) Make sure no corrupted OSPF packet received.
h) Make sure passive interface is not configured
under “router ospf”.
i) Make sure Virtual -link is not configured
over stub area. 

Problem : Route is flapping across the
network in OSPF.
Cause : Link flapping on the router.
Solution:
a) Use summarization on the router to reduce the
impact of flapping route.
b) Use” ip event dampening ” on the
interface.

Problem : OSPF Route is not learnt in OSPF
database. 
Cause: 
1) Originating router not generating LSA.
2) Routes are redistributed in stub areas.
3) LSA filter-list is configured on ABR.

Solution: 
a) Make sure OSPF adjacency is up.
b) Make sure redistribution is used with subnet
keyword as well.
e) Do not redistribute external routes in stub
area.
f) Make sure LSA are not filtered.

Problem : OSPF Route is not installed in routing
table.
Cause :
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) – route 
s        sum and redistribute conn?
8) Backbone area became discontigous (O, OIA,
OE1, OE2)
9) OSPF is enabled on secondary but not on
primary. 

Solution:
a) Make sure distribute-list is configured
correctly.
b) Make sure forwarding address is reachable for
external route.
c) Make sure forwarding address is not even
reachable but also should be learned via inter area and intra area.
c) Make sure network type is correct on both
sides.
d) Make sure there is no backbone
partition. 

Problem : SPF is running  constantly on
OSPF router and CPU HOG.
Cause: 
1) LSA flapping due duplicate router ID/IP
address.
2) Constantly Link flapping   in an
area.

3)All LSA refresh every 30 min (bad!)•4
4)Timers of each LSA get sync (worse!)
5)With group pacing only LSAs that reach max-age
get refreshed periodically

Debug:debug ip ospf monitor
show ip ospf stati
show ip ospf database database-sum

Solution:
a) Use sumarization inter area or external
routes on the router if too many.
b) Use” ip event dampening ” on the
interface.
c) Make sure no duplicate router id is on the
network.
d) Make sure links are stable on the
network.  If links can not stay stable, use summarization. 
c) Interval is configurable

Problem : OSPF Neighbor is flapping between
router.
Cause:
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:
a) Make sure  no high CPU.
b) Interface is stable and no packet drops on
the interface due to error/crc/input error/input queue/output drops.
c) No packet drops on the line card and
platform.

Problem : NSSA ABR not translating Type 7 LSA
Cause:

Solution: 
a) hard code the NSSA type 7 translator by using
command “area 1 nssa translate type7 always”

Problem: GRE Tunnel OSPF adjacency is
flapping
Cause: 
1) Tunnel destination are learning over the
Tunnel.

Solution : 
a) Make sure tunnel destination is not learn
over the OSPF adjacency.

Problem : Sequence number mismatch.
Cause :
1. LSA should one of 5 LSAs
2. If LSA is type 5 and the neighbor is
associated with a stub area
3. If one of the options change
4. If the state of MS bit is inconsistent with
master slave connection
5. If the I-bit is set
6. If the master receives a DBD packet after a
dead interval
7. If the requested LSA is not found, then
something has gone wrong with the database exchange. 

debug ip ospf adjacency  

Solution:

Problem: OSPF: Could not allocate router id
Cause:
1)Common new install problem.
2)If no interface up/up with valid ip address.
3)if no ip addresses assigned.
Solution:
1)Configure a loopback with an ip address.

Problem: OSPF unknown routing protocol
Cause:
1)OSPF is not supported on low end platform
2)For 1000 and 1600 routers download plus
version
3)800 routes are not supported to run ospf

Problem:OSPF not sending hellos on async
interface
Cause:
1)‘async default routing’ is not configured
under the interface

Problem: OSPF not redistributing default
static route
Cause:
1)Need default-information originate to
propagate default

Problem: OSPF-4-ERRRCV msg on the console
Cause:
1)Mismatch area ID, BAD Checksum etc