The Open Shortest Path First (OSPF) protocol makes use of Link State Advertisements (LSAs) to disseminate information throughout each network and to populate the Link State Database (LSDB). LSAs are utilized by routers in order to facilitate the exchange of topology information. An LSA describes a portion of an OSPF network by containing information about its topology and routing. In order to ensure that all routers have the same topology database, routers will continue to learn the whole topology of the network and exchange LSAs with one another. When there is a change to one of OSPF’s links, the protocol really transmits updates, which are known as LSAs. Additionally, LSAs are updated as frequently as every 30 minutes.
The following table lists all of the OSPF LSA types:
|
Types |
Description |
|
LSA Type 1 |
Router LSA |
|
LSA Type 2 |
Network LSA |
|
LSA Type 3 or 4 |
Summary LSA & ASBR LSA |
|
LSA Type 5 |
Autonomous System External LSA |
|
LSA Type 6 |
Multicast OSPF LSA Type |
|
LSA Type 7 |
Defined for Not-So-Stubby-Areas |
|
LSA Type 8 |
External Attribute LSA for BGP |
|
LSA Type 9.10,11 |
Opaque LSA |
Type 1 LSA: Router LSA
- This LSA type is flooded by every router that is located within a certain area.
- Information regarding links that are directly connected is what the LSA is made up of.
- You can identify it by looking for the Router ID or the router that sent it.
- Within that area, flooding occurs, and the LSA Type 1 avoids crossing the ABR.
- In the routing table, these are the “O” routes.
- Every router in the area generates a unique Link ID, and LSA stores all of these IDs.
- The ABR is not crossed by this LSA.
LSA Type 2: Network LSA
- Produced by the DR Router on the broadcast network
- Included in here are the transit routers’ connected lists, as well as their network IDs and subnet masks.
- For every NBMA Network transit broadcast, we can have either a Network LSA or a Type 2 LSA in OSPF.
- Additionally, this LSA may only be flooded within its designated area; it cannot traverse the ABR.
- In the routing table, these are the “O” routes.
- The DR generates this LSA, which contains all segment-attached routers and is local to the area.
LSA Type 3: Summary LSA
- The Type 3 LSA is utilized for the purpose of sharing information about the network with other areas.
- The ABR Router advertises these LSAs.
- In the routing table, these LSA routes are shown as “OIA” routes.
- In order to ensure that the information is properly distributed across the Autonomous System, they are regenerated by the ABRs that come after them.
- LSA 3 – O IA, Network Summary: LSA describes a network from a different area. It is generated by ABR and is propagated from one area to another.
- It has the network ID and the subnet mask.
LSA Type 4: Summary ASBR LSA
- The purpose of this type of LSA, often called a summary LSA, is to inform other areas within the same autonomous system about the ASBR.
- It is produced & generated by the originating area’s ABR.
- The Autonomous System is flooded throughout with these kinds of LSAs.
- Only the ASBR’s router ID will be contained in these LSA.
- The ABR generates LSA 4 – O IA, or Summary ASB Link States, which are then propagated between areas.
LSA Type 5: External LSA
- The ASBR is the sole entity that owns and advertises these.
- Advertisements for networks operated by other autonomous systems are made using these LSAs.
- The whole Autonomous System is flooded throughout with these LSAs.
- During propagation, the advertising router ID remains unchanged across the AS.
- To locate an ASBR, one uses an LSA Type 4.
- As a default, routes are not summarized.
The LSA is created by ASBR and sent via O E1 and O E2.
LSA Type 6: Multicast OSPF LSA
- In the process of routing utilizing multicast, which is the MODPF Routing protocol, these are utilized.
- The Cisco Routers operating system does not support these LSAs.
