LSA

Stands for: Link State Advertisement

The unit of an OSPF link-state database: one router's or one network's local state, flooded to every router in its scope.

Every OSPF router describes its own links in a router-LSA, the designated router of a LAN adds a network-LSA, border routers add summaries and AS boundary routers add external routes. The type sets the flooding scope: one area, one NSSA or the whole AS.

RFC 2328 defines five types. Type 1 router-LSAs and type 2 network-LSAs describe topology and stay inside one area. Type 3 and type 4 summary-LSAs come from area border routers and describe networks and AS boundary routers outside the area. Type 5 AS-external-LSAs are flooded throughout the AS. RFC 3101 adds type 7 for a not-so-stubby area, translated into type 5 at its border, and types 9 to 11 are the opaque LSAs.

Three header fields identify an LSA, LS type, Link State ID and Advertising Router, and three decide which copy is newer: sequence number, checksum and age. Sequence numbers start at 0x80000001. An originator refreshes each of its LSAs every 30 minutes even when nothing has changed, an LSA whose age reaches MaxAge, one hour, is flushed and no longer used, and only the originator may age its own LSAs early.

FRRouting's show ip ospf database lists them by type with age, sequence number and checksum, and its documentation notes that stale LSAs left by a router that vanished without flushing them are not abnormal and do little harm. Forged ones are another matter: RFC 4593 warns that an LSA with the wrong Advertising Router makes every receiver compute an incorrect tree.

Also known as: link state advertisement, link-state advertisement, summary-lsa, type 5 lsa, type 7 lsa

Sources

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