What best describes the type of packets used by OSPF routers to exchange updates on a pointto- point link?

OSPF operates directly over the IP network layer, using IP protocol number 89. On point-to-point links, OSPF routers do not elect a Designated Router (DR) or Backup Designated Router (BDR). Instead, they form an adjacency and exchange all OSPF packets, including Link State Updates (LSUs), by sending them to the reserved AllSPFRouters IP multicast address, 224.0.0.5. This ensures that the neighbor on the other end of the link receives the communication efficiently.

B. Ethernet frames sent to an Ethernet multicast address. This describes the Layer 2 encapsulation, not the Layer 3 packet type used by the OSPF protocol itself, which is the focus of the question.
C. IP packets sent to an IP unicast address. While OSPF can use unicast for specific functions like virtual links or demand circuits, the standard mechanism for exchanging updates on point-to-point links is multicast.
D. UDP packets sent to an IP multicast address. OSPF does not use a transport layer protocol like UDP or TCP. It is encapsulated directly within IP packets, unlike protocols such as RIPv2 which use UDP.

1. Moy, J. (1998). RFC 2328: OSPF Version 2. The Internet Society. Section 1.2, "The OSPF protocol," states, "OSPF runs directly over IP, using IP protocol number 89." Appendix C.2, "Point-to-point networks," further specifies, "All router protocol packets are sent to the multicast address AllSPFRouters, 224.0.0.5."

2. Nokia (2021). 7450 ESS, 7750 SR, 7950 XRS, and VSR Router Configuration Guide, Release 21.10.R1. Document ID: 3HE 17174 AAAC TQZZA 01. In the "OSPF Network Types" section, it is detailed that for the point-to-point network type, "OSPF packets are multicast to 224.0.0.5."

3. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. Chapter 5.2.2, "The OSPF Protocol," explains that OSPF messages are carried directly over IP with an upper-layer protocol number of 89, distinguishing it from protocols that use TCP or UDP.