20 Essential OSPF Interview Questions & Answers .CCNA

What is OSPF and why is it called a link-state protocol?​

Ans: OSPF (Open Shortest Path First) uses the Shortest Path First (SPF) algorithm to determine optimal routes. It’s called link-state because routers share the status of their interfaces (links) to build a complete topology map.

Which OSPF packet types are used in neighbor formation?​

Ans:
Hello - Discover/maintain neighbors
DBD - Summarize LSAs
LSR - Request LSAs
LSU - Send LSAs
LSAck – Acknowledge LSAs

What is the default OSPF Hello and Dead timer on broadcast networks?​

Ans: Hello = 10 seconds, Dead = 40 seconds.

What is an OSPF Area and why is it used?​

Ans: Areas divide the network to reduce SPF calculation load, limit LSDB size, and improve stability.

How is the OSPF Router ID selected?​

Ans:
1. Manually configured Router ID
2. Highest loopback IP
3. Highest active interface IP

What parameters must match for OSPF neighbors to form adjacency?​

Ans: Area ID, subnet/mask, Hello/Dead timers, authentication, network type, and MTU size.

Why might OSPF neighbors be stuck in INIT state?​

Ans: One-way Hello packets or mismatch in parameters like Area ID, timers, or authentication.

Difference between FULL and 2-WAY neighbor states?​

Ans: FULL = Complete LSDB exchange; 2-WAY = Only Hellos exchanged (common with DR/BDR).

Why doesn’t every router form a full adjacency in multi-access networks?​

Ans: To reduce LSA flooding, OSPF elects a DR and BDR; others form adjacency only with them.

What is the role of a DR and BDR?​

Ans: DR is the central point for LSA exchange; BDR takes over if DR fails. Election is based on highest priority, then Router ID.

If DR fails, what happens?​

Ans: BDR becomes DR immediately and a new BDR is elected.

How to influence DR/BDR election?​

Ans: Set OSPF priority using ip ospf priority <0-255>. Priority 0 means never become DR/BDR.

Main OSPF network types & DR/BDR election behavior:​

Ans:
Broadcast - DR/BDR elected
Non-Broadcast - DR/BDR elected
Point-to-Point - No DR/BDR election

Default OSPF network type for serial point-to-point link?​

Ans: Point-to-Point (no DR/BDR election).

What is a passive interface in OSPF?​

Ans: Stops sending Hellos but still advertises the network.
Command:
router ospf 1
passive-interface g0/0

How to advertise a loopback interface?​

Ans:
router ospf 1
network 1.1.1.1 0.0.0.0 area 0

How to prevent OSPF from advertising a network?​

Ans: Exclude it from the network statement or use distribute-list/prefix-list.

What is an LSA? Name 5 types.​

Ans:
LSA = Link-State Advertisement.
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

Difference between Stub, Totally Stubby, and NSSA areas?​

Ans:
Stub: Blocks Type 5 LSAs
Totally Stubby: Blocks Type 3 & 5 LSAs, allows only default route
NSSA: Allows Type 7 LSAs

How does a router handle multiple OSPF areas?​

Ans: It becomes an ABR (Area Border Router), runs OSPF separately per area, and advertises routes using Type 3 LSAs.