OSPF Neighbor States – Complete Guide with Examples

✅ Overview of OSPF Neighbor FSM (Finite State Machine)

The OSPF Neighbor Finite State Machine (FSM) defines the step-by-step process that two OSPF routers go through to form an adjacency.

Build neighbor relationships
Synchronize routing information
Prevent routing loops

✅ Purpose of OSPF Neighbor States

Ensure reliable and ordered exchange of link-state information.
Control the formation of adjacencies only where necessary (e.g., with DR/BDR).
Provide a structured mechanism to detect and recover from neighbor issues.

✅ OSPF Adjacency Process Flow

Routers send Hello packets to discover neighbors.
Routers go through seven neighbor states.
Full adjacency is formed only with DR/BDR (on multi-access networks) or all neighbors (on point-to-point links).

✅ OSPF Neighbor States (in Order)

Order	State
1	Down
2	Init
3	2-Way
4	ExStart
5	Exchange
6	Loading
7	Full

✅ Meaning and Role of Each State

Down: No Hello packets received. Initial state.
Init: Hello received, but our RID not listed yet.
2-Way: Bi-directional Hello exchange; DR/BDR elected here.
ExStart: Master/slave negotiation for DB exchange.
Exchange: DBD packets sent/received.
Loading: Missing LSAs requested and sent.
Full: LSDBs fully synchronized.

✅ OSPF Packet Types and Their Role in States

Packet Type	Purpose	Used In State
Hello	Discover and maintain neighbors	Down, Init, 2-Way
DBD	Describe contents of LSDB	ExStart, Exchange
LSR	Request specific LSAs	Loading
LSU	Send requested LSAs	Loading
LSAck	Acknowledge LSUs	Loading

✅ Neighbor State Transitions

From State	To State	Trigger
Down	Init	Received Hello
Init	2-Way	Neighbor sees our Router ID
2-Way	ExStart	DR/BDR election or P2P
ExStart	Exchange	DBD exchange
Exchange	Loading	Missing LSAs detected
Loading	Full	All LSAs received

✅ Timers (Hello/Dead)

Default Hello Interval: 10s, Dead Interval: 40s

Mismatch prevents adjacency.

✅ DR/BDR Influence

DROTHERs form FULL state only with DR/BDR
DROTHER ↔ DROTHER = 2-Way

✅ Troubleshooting Stuck States

State	Issue Cause
Init	Unidirectional Hello (timers or L2)
ExStart	MTU mismatch, duplicate RIDs, DR/BDR mismatch
Loading	LSA corruption, authentication mismatch

✅ Useful Commands

show ip ospf neighbor
show ip ospf interface
debug ip ospf adj
debug ip ospf hello
show interface <int> | include MTU

✅ Summary Table of OSPF Neighbor States

State	Description	Common Packet Type
Down	No Hello received	—
Init	Hello received, we not seen yet	Hello
2-Way	Bi-directional Hello	Hello
ExStart	Master/slave negotiation	DBD
Exchange	Exchange of LSA headers	DBD
Loading	Requesting LSAs	LSR, LSU, LSAck
Full	Synchronized LSDB	—

OSPF Neighbor States Quiz

1. What is the first OSPF neighbor state when no Hello packets have been received?

A. Down B. Init C. 2-Way D. ExStart

Correct answer is A. Down state means no Hello packets received yet.

2. In which OSPF neighbor state does bidirectional communication occur?

A. ExStart B. Init C. Loading D. 2-Way

Correct answer is D. 2-Way state indicates bidirectional Hello packet exchange.

3. During which state do OSPF routers negotiate master/slave roles?

A. Exchange B. Loading C. ExStart D. Full

Correct answer is C. ExStart is when routers decide who is master or slave.

4. What packet type is used during the Exchange state to describe the LSDB contents?

A. Hello B. Database Description (DBD) C. Link State Request (LSR) D. Link State Update (LSU)

Correct answer is B. DBD packets list LSDB contents during Exchange state.

5. Which OSPF neighbor state requests missing LSAs using LSR packets?

A. Loading B. Exchange C. Full D. Init

Correct answer is A. Loading state uses LSR packets to request missing LSAs.

6. In broadcast networks, why do DROTHER routers remain in the 2-Way state with each other?

A. They do not send Hello packets B. They are not on the same subnet C. They are not allowed to exchange routing information D. To prevent unnecessary LSA flooding and adjacencies

Correct answer is D. DROTHER routers stay in 2-Way state with each other to minimize overhead.

7. What causes OSPF neighbors to get stuck in the ExStart state?

A. Authentication mismatch B. MTU mismatch or duplicate Router IDs C. No Hello packets received D. Link down

Correct answer is B. MTU mismatch and duplicate RIDs can cause ExStart to be stuck.

8. Which command shows detailed OSPF neighbor information including state and DR/BDR?

A. show ip ospf neighbor B. show ip route ospf C. show ip protocols D. show ip ospf database

Correct answer is A. This command lists neighbors with their OSPF states and roles.

9. What is the default Hello interval on a broadcast network?

A. 5 seconds B. 40 seconds C. 10 seconds D. 30 seconds

Correct answer is C. The default Hello interval on broadcast networks is 10 seconds.

10. What OSPF neighbor state indicates that routers are fully adjacent and databases synchronized?

A. Loading B. Full C. Exchange D. 2-Way

Correct answer is B. Full state means adjacency is complete and LSDBs are synchronized.

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