Core Issue
When Open Shortest Path First (OSPF) is enabled on a router or when a router configured for OSPF is powered up, it tries to discover its OSPF neighbors and synchronize its database with them. Routers are said to be OSPF neighbors when they see their router ID in the received hello packet and the status of their OSPF relationship transitions to Two-way.
OSPF neighbors then exchange routing information with each other. Once their databases are updated and synchronized, the status of their OSPF relationship changes to Full. However, the status of the OSPF relationship between OSPF neighbors does not always transition to Full and might remain stuck in Two-way instead.
Resolution
This is expected behavior on routers running OSPF. OSPF routers on multi-access segments synchronize their databases with their Designated Router (DR) or a Backup Designated Router (BDR) only. The OSPF status between two routers transitions to Full only if at least one of them is a DR or a BDR. The state remains Two-way between a pair of routers if both are drothers.
For more information on the relationship between OSPF routers stuck in the two-way state, refer to Why Does the show ip ospf neighbor Command Reveal Neighbors Stuck in Two-Way State?
Troubleshooting Commands:
1) show ip ospf neighbors....This command will show you states of your neighbor and dead timer.
Example:
Router#sh ip os ne
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DR 00:00:38 10.1.1.1 FastEthernet0/0
1) Show ip ospf int brief ...This command will give you OSPF property briefly on each interface.
Example:
Router#sh ip os int bri
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Fa0/0 100 0 10.1.1.2/24 10 BDR 1/1
3) Show ip ospf int <inteface no>...This command will give detail OSPF interface information like network-type,authentication type and key, interface mask,cost etc.
Example:
Router#sh ip os int fa0/0
FastEthernet0/0 is up, line protocol is up
Internet Address 10.1.1.2/24, Area 0
Process ID 100, Router ID 10.1.1.2, Network Type BROADCAST, Cost: 10
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 10.1.1.1
Backup Designated router (ID) 10.1.1.2, Interface address 10.1.1.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:06
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 1.1.1.1 (Designated Router)
Suppress hello for 0 neighbor(s)
4) Sh int (interface-no>....This command will give you physical link parameter like link state,MTU(Which is imp factor in ospf while exchanging database and need to same on both site>.
Example:
Router#sh int fa0/0 | in MTU | up
FastEthernet0/0 is up, line protocol is up
MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,
For MTU ,you need to check on (Routing) IP MTU that's will gonna stuck your OSPF process in ex-start state if both side device don't have same MTU , default both MTUs are same on IOS platform but you can change using "ip mtu" command under interface on the router and on Switch ,you can set system routing MTU.
Example:
Router#sh ip int fa0/0 | in MTU
MTU is 1400 bytes
