This tutorial explains what an LSA is, LSA types, LSA data structure, LSA flooding, and how OSPF uses an LSA to exchange network information.
What is an LSA?
An LSA (Link State Advertisement) is a data packet that describes a specific part of the OSPF network. There are 11 types of LSA. OSPF routers use each type of LSA to share or exchange different information. Since an LSA describes a part of the OSPF network, each OSPF router learns all LSAs that it needs to function and stores them into the LSDB (Link-State Database).
In other words, an LSA is an OSPF data packet that contains some specific information about network topology. The LSDB is the collection of all LSA packets the router knows. The state, when all OSPF routers on the OSPF network learn all required LSAs is called convergency.
LSA flooding
LSA flooding is a process that OSPF routers use to share and learn all required LSAs. In the process of LSA flooding, all routers collectively advertise all known LSAs to all the other routers. At the end of this process, every router on the network has every required LSA.
After this process, if any information changes, the affected router creates an LSA describing the change and floods the LSA into the network. Each recipient validates the LSA update and sends an acknowledgment back to the sending router that confirms that it received the flooded update.
Based on the network type, OSPF routers use the following addresses to flood LSAs.
- In a point-to-point network, they use the reserved multicast address 224.0.0.5.
- In a broadcast network, they use the multicast address 224.0.0.6.
- In a point-to-multipoint network, they use the adjacent router’s unicast IP address.
Types of LSAs
Instead of using the same LSA type for all purposes, OSPF routers use a different type of LSA for each purpose. They use 11 types of LSAs. The following table describes each type of LSA and its purpose.
| Type | Name | Originated by | Distance | Description |
| 1 | Router LSA | by all routers within the area | Within the area | Contain information about RID and directly connected interfaces. |
| 2 | Network LSA | DR (Designated router) | Within the area | Contain information about all paths in the network. |
| 3 | Summary LSA | ABR (Area Border Router) | Within the network | Contain summarized information about all paths in the area |
| 4 | ASBR Summary | ABR (Area Border Router) | Within the network | Contain information about the ASBR where the type 5 LSA originated. |
| 5 | AS-external LSA | ASBR (Autonomous System Boundary Router) | Within the network | Contain information about routes imported into OSPF from other routing protocols |
| 6 | Group Membership LSA | It was defined for multicast extensions. It has been deprecated and is not currently used. | ||
| 7 | NSSA External LSA | ASBR (Autonomous System Boundary Router) | Intra-area | Identical to type-5 but are flooded within the NSSA only. Allow routers in NSSA to send external routing information for redistribution. |
| 8 | Link-local LSA (OSPFv3) | by all routers | within the area Link | OSPFv3 uses it to share information about link-local addresses and a list of IPv6 addresses on the link. |
| 9 | Link-local opaque (OSPFv2) | Link-local | In OSPFv2, it contains extended information about a specific application | |
| 9 | Intra-Area-Prefix (OSPFv3) | Link-local | In OSPFv3, it contains prefixes for stub and transit networks in the link-state ID. | |
| 10 | Area-local opaque | Area-local | Contain information that should be flooded by other routers even if the router is not able to understand the extended information itself. | |
| 11 | Autonomous System opaque | Within the network | Identical to type-10 but are not flooded into a stub area. |
The opaque LSA types are designed for application-specific purposes. For example, an application can use them to flood bandwidth information. Opaque LSA types are 9,10, and 11. Each type of LSA has a different flooding scope.
Important LSA types for the CCNA exam
If you are learning LSA types for CCNA Routing and Switching exam, you should focus only on LSA types 1, 2, and 3. The remaining LSA types are not covered in the CCNA exam syllabus. The CCNP exam covers them. Since the CCNA exam syllabus includes OSPF topics related to LSA types 1, 2, and 3, we will discuss these types in detail.
Before we learn these types, we need to understand the OSPF area concept and roles of IR, ABR, and DR routers in the OSPF network.
Basic of OSPF area concept
OSPF uses a hierarchical design to control network traffic. In the hierarchical design, OSPF uses two levels. In OSPF terminology, a hierarchical level is called an area. There are types of OSPF areas: backbone area or area 0 and area-off backbone.
Backbone area
The backbone area is the central point of this implementation. Routers running in this area are required to maintain a complete database of the entire network. All areas need to connect with this area through a physical link or a virtual link.
Area off backbone
The area-off backbone is an extension of the backbone area. Routes running in this area maintain an area-specific database instead of the complete database. This feature reduces network traffic and speeds up the convergence process.
Roles of IR, ABR, and DR routers in an OSPF network
IR, ABR, and DR routers use LSA 1, LSA 2, and LSA 3 for many purposes. To understand these types in detail, we have to understand the role of IR, ABR, and DR routers in an OSPF network.
IR (Internal Router)
An IR router has the least responsibilities in the OSPF network. It maintains only area-specific routing information.
ABR (Area Border Router)
An ABR connects the area to another area. It keeps the area-specific routing information and summarized routing information of the entire network.
DR (Designated Router)
In Ethernet/Broadcast environment, IR routers don't share routing information directly. They elect a DR router. The DR router is responsible for maintaining, sharing, and updating area-specific routing information.
The following image shows an example OSPF network.
LSA type 1
An LSA type 1 message is called router LSA. OSPF routers use LSA type 1 messages to advertise their RID and information about directly connected networks. A RID is the unique ID of the router in the OSPF network. Since all routers running in the area need this information about the other routers, all OSPF routers generate and advertise LSA type 1 messages. LSA type 1 messages do not cross the area boundary. They remain within the area.
LSA type 2
An LSA type 2 message is known as network LSA. As mentioned earlier, in Ethernet/broadcast environment, routers do not share routing information directly. They elect a DR, and then the DR shares the routing information. To share routing information, the DR uses LSA type 2 messages. LSA type 2 messages also do not cross the area boundary. They stay within the area.
LSA type 3
An LSA type 3 message is called summary LSA. ABR routers use LSA type 3 messages to exchange routing information with the other ABR routers. By default, LSA type 3 messages contain detailed routing information. But if required, an administrator can instruct the ABR router to summarize the routing information before sharing it with another ABR. For this, the administrator can use the summary command.
That’s all for this tutorial. In this tutorial, we discussed what LSA is, LSA types, the meaning of each LSA type, and the information LSA type 1, 2, and 3 carry.