Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 122
Message Types

OSPF routers pass messages to each other in the form of Link-State Advertisements (LSAs). Each link-state advertisement describes a piece of the OSPF routing domain. All link-state advertisements are then flooded throughout the OSPF routing domain, but within a single area. A single area can be an entire OSPF domain. The flooding algorithm is reliable, ensuring that all routers have the same collection of link-state advertisements.

Type 1—Router Links Advertisement: This message is flooded within an area and contains information about neighbors’ router links (basically the IP address of an interface and the cost associated with that interface). Every router originates a router links advertisement.

Type 2—Network Links Advertisement: This message is flooded within an area. It is generated by the designated router (DR) and includes information on all routers on this multiaccess network. Whenever the router is elected the DR, it originates a network links advertisement.

Type 3—Summary Links Advertisement: Flooded into an area by an Area Border Router (ABR). This message describes reachable networks from outside the area (in other areas of the OSPF domain).

Type 4—AS Boundary Router Summary Link Advertisement: This message is flooded into an area by an ABR. The message describes the cost from this router to an AS Boundary Router.

Type 5—AS External Link Advertisement:

This message is flooded to all areas except stub areas (explained later). It describes an external network reachable via the AS Boundary Router that generated it.

Type 6—Multicast Group Membership LSAs: Allows multicast-enabled OSPF router to distribute IGMP (multicast group information).

One last thing about LSAs: They contain 32-bit sequence numbers. This number is used to detect old and duplicate LSA packets. Each new LSA uses an incremented sequence number; therefore, OSPF routers keep their LSA databases current by updating them with an LSA of a higher sequence number. This also allows the OSPF router to flush out old entries.

Another method employed by OSPF on its LSA database is the age field. Each LSA entry has an expiration timer that can expire, allowing the database to purge old entries.