Learn how OSPF describes an area, identifies LSA origins, distributes link-state information, and treats external routes.
What you will be able to do
- Identify the main LSA types held in an OSPF area link-state database.
- Distinguish the roles of router-LSAs, network-LSAs, summary-LSAs, and AS-external-LSAs.
- Interpret the Advertising Router and Link State ID fields in common OSPF descriptions.
- Explain how OSPF floods LSAs within an area and across the routing domain.
01
Why OSPF Uses LSAs
OSPF builds its view of the network from link-state advertisements, or LSAs. Each advertisement describes a particular part of the routing environment so routers can maintain a shared link-state database.
The useful distinction is scope. Some LSA information belongs to one area, while AS-external information can travel across the entire autonomous system. This scope helps organize what routers learn and where they learn it.
02
The Area Link-State Database
An area link-state database contains router-, network-, and summary-LSAs in its area-specific structure. It also includes AS-external-LSAs supplied from the global structure.
Think of the database as the router’s collected description of relevant OSPF information. The area portion describes local structure, while external entries represent information whose flooding scope extends beyond one area.
03
Router-LSAs
A router-LSA is generated by every router in an OSPF area. It is therefore the LSA type associated with an individual router’s participation in that area.
For example, if an area contains routers with IDs R1, R2, and R3, each router generates its own router-LSA. The database can then contain one router description from each of those routers.
04
Network-LSAs
A network-LSA lists the routers that are fully adjacent to the Designated Router. Each listed router is identified by its OSPF Router ID.
This differs from a router-LSA. A router-LSA is generated by each router, whereas a network-LSA describes the routers fully adjacent to the Designated Router.
05
Summary-LSAs
Area border routers originate summary-LSAs. These advertisements belong to the terminology of information exchanged through area boundaries.
A default summary-LSA has two notable values. Its Link State ID is DefaultDestination, and its metric uses the per-area StubDefaultCost.
06
AS-External-LSAs
AS-external-LSAs are the only LSA type flooded throughout the entire autonomous system. They are included in an area’s database through the global structure.
All other LSA types are limited to a single area. Therefore, AS-external-LSAs have broader scope than router-, network-, and summary-LSAs.
07
Reading LSA Identity Fields
The Advertising Router field identifies the OSPF Router ID of the router that originated an LSA. This field answers the question, “Which OSPF router created this advertisement?”
For router and network descriptions, the Link State ID identifies the OSPF Router ID of the router being described. A default summary-LSA is different: its Link State ID is the special value DefaultDestination.
08
How LSAs Move
By default, OSPF floods a new LSA across interfaces in the same area, except through the interface on which the LSA was received. This prevents the advertisement from simply returning through its incoming path.
Link State Update packets carry LSAs through the OSPF routing domain. Each such packet moves new LSAs one hop farther from their point of origin, extending their distribution step by step.
09
Lesson Recap: Ready for Practice and Prove
Every router in an OSPF area generates a router-LSA, while an area border router originates summary-LSAs. A network-LSA identifies routers fully adjacent to the Designated Router.
The area link-state database contains router-, network-, and summary-LSAs in its area structure. It also includes AS-external-LSAs from the global structure.
The Advertising Router field names the originating router through its OSPF Router ID. For router and network descriptions, the Link State ID identifies the described router’s OSPF Router ID.
OSPF normally floods new LSAs through same-area interfaces other than the receiving interface. Link State Update packets carry them one hop farther from their origin.
AS-external-LSAs reach the entire autonomous system, while other LSA types stay within one area. Use these scope, role, and identity distinctions in Practice and Prove.