Link State Databases 475Link StateDatabasesOSPF routers use the information that is contained in the link stateadvertisements (LSAs) to build and maintain link state databases. Eachlink state database contains the link state advertisements fromthroughout the areas to which the router is attached. OSPF uses thefollowing types of LSAs:n Router Link State Advertisementsn Network Link State Advertisementsn Summary Link State Advertisementsn External Link State AdvertisementsRouter Link StateAdvertisementsAll routers in an OSPF area originate router link state advertisements, alsoknown as link state advertisements. Each link state advertisementdescribes the state and cost of the originating router’s links (interfaces) tothe area. Information contained in each link state advertisement includes:n LSID (Link State ID) — The ID of the router that generated the LSA.n Router ID — ID of the router that originated the LSA.n LS Seq (Link State Sequence) — The sequence number of theadvertisement. Used to detect old or duplicate link stateadvertisements.n LS age — The time, in seconds, since the LSA was generated.n Flags — Possible values:n V — Router is the endpoint of an active virtual link that is using thearea as a transit area.n ASBR — Router is an autonomous system boundary router (ASBR).n ABR — Router is an area border router (ABR).n Link Type — A description of the router link. Possible values:n PTP — Connection is point-to-point to another router.n Transit — Connection is to a transit network.n Stub — Connection to a stub network.n Virtual link — Connection is to a far-end router that is theendpoint of a virtual link.
