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CCNA - OSPF Deep Dive

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Question 1

?? Multiple Choice (Easy)

What type of routing protocol is OSPF?

Answer choices

  • A. Distance vector

  • B. Path vector

  • C. Link state (Correct)

  • D. Hybrid

Explanation

OSPF (Open Shortest Path First) is a link-state routing protocol. Each router builds a complete topology map (LSDB — Link State Database) and independently runs the Dijkstra SPF algorithm to compute shortest paths. Unlike distance vector protocols (RIP), OSPF routers don't share routing tables — they share Link State Advertisements (LSAs) describing network topology.

Question 2

?? True/False (Hard)

OSPFv3 uses IPv6 next-hop addresses but can also run over IPv4 using address-family support.

Answer choices

  • A. True (Correct)

  • B. False

Explanation

OSPFv3 was originally designed for IPv6 but was extended with address-family support (RFC 5838) to run both IPv4 and IPv6 over a single OSPFv3 process. OSPFv3 key differences from OSPFv2: runs directly over IPv6 link-local addresses; uses IPv6 extension headers; removed IP addressing from Hello/LSA (addressing is address-family dependent); router-id is still a 32-bit value.

Question 3

?? Fill in the Blank (Easy)

OSPF uses multicast address [[blank1]] to send Hello packets to all OSPF routers on a segment.

Explanation

OSPF uses two multicast addresses: 224.0.0.5 (AllSPFRouters — sent to all OSPF routers); 224.0.0.6 (AllDRouters — sent only to DR and BDR). OSPF operates over IP directly (protocol 89, not UDP/TCP). On broadcast segments, the DR uses 224.0.0.5 to send LSU updates to all routers.

Question 4

?? True/False (Easy)

The OSPF Router ID must be manually configured and cannot be automatically determined.

Answer choices

  • A. True

  • B. False (Correct)

Explanation

OSPF RID selection (highest priority to lowest): 1) Manually configured router-id command; 2) Highest IP on any loopback interface; 3) Highest IP on any active non-loopback interface. Best practice is to manually configure a loopback with a unique IP and set it as the router-id for stability (physical interface IPs can change).

Question 5

?? Multiple Choice (Medium)

What is the OSPF Hello interval on a broadcast (Ethernet) interface, and what is the Dead interval?

Answer choices

  • A. Hello: 30 sec, Dead: 120 sec

  • B. Hello: 10 sec, Dead: 40 sec (Correct)

  • C. Hello: 5 sec, Dead: 20 sec

  • D. Hello: 60 sec, Dead: 240 sec

Explanation

OSPF default timers on broadcast/point-to-point: Hello = 10 seconds, Dead = 40 seconds (4x Hello). On NBMA: Hello = 30 sec, Dead = 120 sec. Both ends of an adjacency must have matching Hello and Dead intervals. Dead interval = time before declaring a neighbor down. Timers can be tuned for faster convergence.