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Getting ready for your CCNA or network engineer interview? 
Then you must know OSPF (Open Shortest Path First) inside and out.
Here’s your complete OSPF Q&A cheat sheet with clear answers, examples, and real-world explanations.
Answer:
OSPF (Open Shortest Path First) uses the Shortest Path First (SPF) algorithm (Dijkstra’s Algorithm) to find the best path.
It’s called link-state because routers exchange information about their interfaces (links) to build a complete topology map of the network.
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OSPF routers use five key packet types to form and maintain adjacencies:
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An area is a logical grouping that helps reduce SPF calculations, limits the Link-State Database (LSDB) size, and improves network stability.
Example: Area 0 (Backbone Area) connects all other areas.
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Occurs due to one-way Hello packets or mismatched settings (like Area ID, authentication, or timers).
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To reduce LSA flooding on multi-access networks, OSPF elects DR (Designated Router) and BDR (Backup Designated Router) - other routers form adjacency only with them.
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To set priority:
(Priority 0 = never become DR/BDR)
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Point-to-Point - no DR/BDR election needed.
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A passive interface stops sending Hello packets but still advertises the network.
Example:
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Exclude it from the network command or use a prefix-list or distribute-list filter.
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LSA (Link-State Advertisement) - carries topology info in OSPF.
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They act as ABRs (Area Border Routers), running OSPF separately per area and sharing inter-area routes using Type 3 LSAs.
Mastering OSPF isn’t just about memorizing commands - it’s about understanding how routers talk to each other 
.
These 20 questions give you a real CCNA-level foundation for both interviews and real-world troubleshooting.
Comment “SPF Master” if you learned something new!
Share this with your study group or lab partner!
Follow TabCode for more networking, CCNA, and cybersecurity insights!
Then you must know OSPF (Open Shortest Path First) inside and out.
Here’s your complete OSPF Q&A cheat sheet with clear answers, examples, and real-world explanations.
What is OSPF and Why Is It a Link-State Protocol?
Answer:OSPF (Open Shortest Path First) uses the Shortest Path First (SPF) algorithm (Dijkstra’s Algorithm) to find the best path.
It’s called link-state because routers exchange information about their interfaces (links) to build a complete topology map of the network.
OSPF Packet Types in Neighbor Formation
Answer:OSPF routers use five key packet types to form and maintain adjacencies:
- Hello: Discover/maintain neighbors
- DBD: Summarize LSAs
- LSR: Request LSAs
- LSU: Send LSAs
- LSAck: Acknowledge LSAs
Default OSPF Timers
Answer:- Hello Timer: 10 seconds
- Dead Timer: 40 seconds (4x Hello)
These are defaults for broadcast and point-to-point networks.
What is an OSPF Area?
Answer:An area is a logical grouping that helps reduce SPF calculations, limits the Link-State Database (LSDB) size, and improves network stability.
Example: Area 0 (Backbone Area) connects all other areas.
How is the OSPF Router ID Selected?
Answer:- Manually configured Router ID
- Highest loopback IP
- Highest active interface IP
Code:
show ip ospf
Parameters Required for Neighbor Adjacency
Answer:- Area ID
- Subnet & mask
- Hello/Dead timers
- Authentication
- Network type
- MTU size
Why Neighbors Get Stuck in INIT State
Answer:Occurs due to one-way Hello packets or mismatched settings (like Area ID, authentication, or timers).
FULL vs 2-WAY State
Answer:- FULL: Routers have fully exchanged LSDBs.
- 2-WAY: Only Hellos exchanged (common on multi-access networks with DR/BDR).
Why Not All Routers Form Full Adjacencies?
Answer:To reduce LSA flooding on multi-access networks, OSPF elects DR (Designated Router) and BDR (Backup Designated Router) - other routers form adjacency only with them.
DR & BDR Roles Explained
Answer:- DR: Central point for LSA exchange.
- BDR: Standby, takes over if DR fails.
- Election: Based on highest OSPF priority, then Router ID.
To set priority:
Code:
ip ospf priority <0-255>
OSPF Network Types & DR/BDR Behavior
| Network Type | DR/BDR Election |
|---|---|
| Broadcast |  Yes |
| Non-Broadcast | Yes |
| Point-to-Point |  No |
Default OSPF Network Type for Serial Links
Answer:Point-to-Point - no DR/BDR election needed.
What is a Passive Interface?
Answer:A passive interface stops sending Hello packets but still advertises the network.
Example:
Code:
router ospf 1
passive-interface g0/0
Advertising a Loopback Interface
Answer: Code:
router ospf 1
network 1.1.1.1 0.0.0.0 area 0
Prevent OSPF from Advertising a Network
Answer:Exclude it from the network command or use a prefix-list or distribute-list filter.
What is an LSA? Name 5 Types
Answer:LSA (Link-State Advertisement) - carries topology info in OSPF.
- Type 1: Router LSA
- Type 2: Network LSA
- Type 3: Summary LSA
- Type 4: ASBR Summary LSA
- Type 5: External LSA
- Type 7: NSSA External LSA
Stub, Totally Stubby & NSSA Areas
| Type | Blocks | Allows |
|---|---|---|
| Stub | Type 5 LSAs | Type 3, default route |
| Totally Stubby | Type 3 & 5 LSAs | Only default route |
| NSSA | External routes via Type 7 LSAs | - |
Routers in Multiple Areas
Answer:They act as ABRs (Area Border Routers), running OSPF separately per area and sharing inter-area routes using Type 3 LSAs.
Bonus: OSPF Command Examples
View OSPF Neighbors:
Code:
show ip ospf neighborReset OSPF Process:
Code:
clear ip ospf processConfigure OSPF on a Router:
Code:
router ospf 1
network 10.0.0.0 0.0.0.255 area 0 Final Thoughts
Mastering OSPF isn’t just about memorizing commands - it’s about understanding how routers talk to each other .These 20 questions give you a real CCNA-level foundation for both interviews and real-world troubleshooting.
Comment “SPF Master” if you learned something new! Share this with your study group or lab partner! Follow TabCode for more networking, CCNA, and cybersecurity insights! Last edited: