What is the source IP address in an OSPF update?

OSPF packets, including updates (Link State Updates or LSUs), are encapsulated within standard IP datagrams. According to RFC 2328, which defines the OSPFv2 protocol, the source IP address in the IP header of an OSPF packet is always the unicast IP address of the router's interface from which the packet is being transmitted. The destination address is typically the multicast address 224.0.0.5 (AllSPFRouters) or 224.0.0.6 (AllDRouters), or a unicast address in specific cases like on point-to-point links or virtual links. This ensures proper IP-level communication on the local network segment.

A. 224.0.0.5 is the AllSPFRouters multicast destination address used to send OSPF packets to all OSPF-speaking routers on a segment; it is never used as a source address.
B. 224.0.0.6 is the AllDRouters multicast destination address used by Designated Routers (DRs) and Backup DRs (BDRs) to send updates; it is never used as a source address.
D. The system address of a Nokia router is typically used to derive the OSPF Router ID, which is a field within the OSPF packet payload, not the source IP address in the IP header.

1. Moy, J. (1998). OSPF Version 2. RFC 2328. The Internet Society.

Reference: Appendix C.1, "IP Packet Headers".

Quote: "The Source IP address is set to the IP address of the sending interface." This section explicitly states that the source IP address for an OSPF packet is the address of the outgoing interface.

2. Nokia (2021). 7450 ESS, 7750 SR, 7950 XRS, and VSR Router Configuration Guide, Release 21.10.R1.

Reference: Chapter "OSPF", Section "OSPF Areas and Interfaces".

Content: The guide details the configuration of IP addresses on interfaces that participate in OSPF. It states, "An interface is associated with a single IP address and mask...". This configured address is the one used for all IP communication originating from that interface, including OSPF packets, aligning with the standard defined in RFC 2328.