How Does OSPF Communicate Routing Updates?

N10-009 By islever Editorial Last updated

Short answer

C Multicast

Question

Which of the following does OSPF use to communicate routing updates?
  1. Unicast
  2. Anycast
  3. Multicast Correct Answer
  4. Broadcast

Key Takeaways

The question tests your understanding of OSPF link-state operation, where updates are sent to a controlled multicast group rather than broadcast to every device; the trap is selecting broadcast because many older routing protocols flood routing tables to all hosts.

OSPF uses multicast—specifically 224.0.0.5 for all OSPF routers and 224.0.0.6 for designated routers—to exchange routing updates efficiently. In the CompTIA Network+ N10-009 exam, the correct choice is C, multicast, as strongly confirmed by the community vote and comments.

Common mistake: Choosing D, Broadcast, is the most common error: OSPF is often confused with distance-vector flooding or with how RIP sends updates, but OSPF uses well-known multicast addresses to ensure only OSPF-enabled routers process the updates.

Expert Analysis

Why the Answer Is Correct

OSPF is a link-state routing protocol that forms neighbor adjacencies and sends Hello packets, followed by link-state advertisements, to reserved multicast groups. The standard multicast addresses are 224.0.0.5 (AllSPFRouters) and 224.0.0.6 (AllDRouters), as the top community comment correctly explains. Using multicast allows OSPF to share routing information only with routers that participate in OSPF, avoiding unnecessary load on hosts and non-OSPF devices.

Why the Other Options Are Wrong

A, Unicast, is not OSPF's default update delivery method; unicast is only used in special non-broadcast multi-access configurations. B, Anycast, is an IPv6/IP addressing strategy where multiple devices share an address for redundancy, and it is not how OSPF communicates. D, Broadcast, would send packets to every host on the local network segment, which OSPF deliberately avoids. Instead, OSPF uses multicast so only routers listening to the 224.0.0.5 or 224.0.0.6 groups receive the updates.

Community Comment Notes

The most useful comment identifies the exact multicast addresses and explains that multicast efficiently reaches only OSPF-configured routers. Other comments simply reaffirm 'C. Multicast,' making the answer clear. One low-rated comment mentions 'Garrett says D,' but that contradicts the RFC 2328 standard and OSPF's normal behavior. The overall consensus—including the 88-vote community tally—is that multicast is the correct answer.

Official Reference

These links open the official vendor documentation in a new tab.

Exam Strategy

Memorize OSPF's key multicast addresses: 224.0.0.5 for all OSPF routers and 224.0.0.6 for designated routers. On exam day, eliminate 'broadcast' immediately because OSPF sends only to a group of interested multicast listeners, not to every node on the network segment.

Community Votes

C
100%

100% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 4 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 4

  1. ba10f26 👍 4 Answered C
    The correct answer is: C. Multicast OSPF (Open Shortest Path First) uses multicast to communicate routing updates with other routers. Specifically, it sends updates to the following multicast addresses: 224.0.0.5: All OSPF routers 224.0.0.6: All OSPF designated routers (DRs) Using multicast allows OSPF to efficiently share routing information only with routers that are configured to participate in the OSPF process, rather than broadcasting to all devices on the network.
  2. HeatSquad77 👍 2 Answered C
    C. multicast Method of sending a packet in which the sending computer sends it to a group of interested computers.
  3. efe498c 👍 1 Answered C
    The answer is OSPF uses multicast to communicate routing updates.
  4. lord_darth_vader 👍 1 Answered D
    Garrett says its D

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