The link-state advertisement (LSA) is a basic communication means of the OSPF routing protocol for the Internet Protocol (IP). It communicates the router's local routing topology to all other local routers in the same OSPF area. OSPF is designed for scalability, so some LSAs are not flooded out on all interfaces, but only on those that belong to the appropriate area. In this way detailed information can be kept localized, while summary information is flooded to the rest of the network. The original IPv4-only OSPFv2 and the newer IPv6-compatible OSPFv3 have broadly similar LSA types.

The LSA types defined in OSPF are as follows:

The opaque LSAs, types 9, 10, and 11, are designated for upgrades to OSPF for application-specific purposes. For example, OSPF-TE has traffic engineering extensions to be used by RSVP-TE in Multiprotocol Label Switching (MPLS). Opaque LSAs are used to flood link color and bandwidth information. Standard link-state database (LSDB) flooding mechanisms are used for distribution of opaque LSAs. Each of the three types has a different flooding scope.

For all types of LSAs, there are 20-byte LSA headers. One of the fields of the LSA header is the link-state ID.

Each router link is defined as one of four types: type 1, 2, 3, or 4. The LSA includes a link ID field that identifies, by the network number and mask, the object that this link connects to.

Depending on the type, the link ID has different meanings as shown in below table:

As per Appendix-A.3.1 of RFC 2328, all OSPF packets start with a common LSA "24-byte header" as shown below.

For