When a router sends a routing protocol packet, it fills in the fields of the standard OSPF packet header as follows. For more details on the header format consult Section A.3.1:
Set to 2, the version number of the protocol as documented in this specification.
The type of OSPF packet, such as Link state Update or Hello Packet.
The length of the entire OSPF packet in bytes, including the standard OSPF packet header.
The identity of the router itself (who is originating the packet).
The OSPF area that the packet is being sent into.
The standard IP 16-bit one's complement checksum of the entire OSPF packet, excluding the 64-bit authentication field. This checksum should be calculated before handing the packet to the appropriate authentication procedure.
Each OSPF packet exchange is authenticated. Authentication types are assigned by the protocol and documented in Appendix D. A different authentication scheme can be used for each OSPF area. The 64-bit authentication field is set by the appropriate authentication procedure (determined by AuType). This procedure should be the last called when forming the packet to be sent. The setting of the authentication field is determined by the packet contents and the authentication key (which is configurable on a per- interface basis).
The IP destination address for the packet is selected as follows. On physical point-to-point networks, the IP destination is always set to the address AllSPFRouters. On all other network types (including virtual links), the majority of OSPF packets are sent as unicasts, i.e., sent directly to the other end of the adjacency. In this case, the IP destination is just the Neighbor IP address associated with the other end of the adjacency (see Section 10). The only packets not sent as unicasts are on broadcast networks; on these networks Hello packets are sent to the multicast destination AllSPFRouters, the Designated Router and its Backup send both Link State Update Packets and Link State Acknowledgment Packets to the multicast address AllSPFRouters, while all other routers send both their Link State Update and Link State Acknowledgment Packets to the multicast address AllDRouters.
Retransmissions of Link State Update packets are ALWAYS sent as unicasts.
The IP source address should be set to the IP address of the sending interface. Interfaces to unnumbered point-to-point networks have no associated IP address. On these interfaces, the IP source should be set to any of the other IP addresses belonging to the router. For this reason, there must be at least one IP address assigned to the router.[2] Note that, for most purposes, virtual links act precisely the same as unnumbered point-to-point networks. However, each virtual link does have an IP interface address (discovered during the routing table build process) which is used as the IP source when sending packets over the virtual link.
For more information on the format of specific OSPF packet types, consult the sections listed in Table 10.
Type Packet name detailed section (transmit)
_________________________________________________________
1 Hello Section 9.5
2 Database description Section 10.8
3 Link state request Section 10.9
4 Link state update Section 13.3
5 Link state ack Section 13.5
Table 10: Sections describing OSPF protocol packet transmission.