Understanding OSPF LSA, Its Types and Functions

In this blog, we will understand the OSPF LSA types and how OSPF uses (LSAs) are exchanged, including how the information is exchanged in the network topology between routers. 

A router's Link-State Database stores LSAs that are received (LSDB). The Shortest Path First (SPF) algorithm is used by OSPF to determine the optimum routes for each network once the LSDBs between routers are in sync. It is crucial to realise that LSAs are route-related data sent inside Link State Update (LSU) packets. 

Therefore, it is essential for any network engineer to comprehend how LSAs are kept within an LSU. Let's dig deep into OSPF LSA types, but before that, we must have a basic understanding of the functions of LSAs in OSPF. 

Functions of LSAs in OSPF

To create multiple paths throughout the network, OSPF uses LSA exchanges in the following activities: 

1. Discovering neighbors 

2. Electing a DR and BDR 

3. Establishing adjacencies and synchronizing databases 

LSAs are the building blocks of the OSPF LSDB. Individually, LSAs act as database records, and when combined, they describe the entire topology of an OSPF area. There are several types of OSPF network LSAs, but not all are in use. 

OSPF LSA Types

There are basically 7 OSPF LSA types which are as follows: 

Let's discuss each type one by one. 

OSPF Router LSA – Type 1

IRs generate these types of LSA. They carry the information of all directly connected routes, and it always stays within the area. You will find the router-id of the originating router in the link-state id field. Once routing is done on the OSPF router, it generates the Type-1 LSA; you can use this command to verify the LSA. 

#SHOW IP OSPF DATABASE 

And you'll get an LSA Type-1 like this in the database table of OSPF. 

Network LSA – Type 2

The DR generates it on a multi-access network segment and always stays within the area when the neighborship is formed between two routers. Same you can verify in the database table of OSF. 

Summary LSA

Using a summary prefix, the ABRs produce it into all other areas in order to summarise the area to which it is directly connected and to promote inter-area router information to other areas to which it is connected. 

Summary LSAs are further classified into 2 types: 

1. Type 3 LSA 

2. Type 4 LSA 

Type 3 summary LSA 

Routes to destinations in other areas within the OSPF network, i.e., inter-area destinations (O-IA). Basically, when multi-areas in OSPF are configured then, only this LSA type is generated in OSPF. 

Type 4 summary LSA 

LSA Type 5 (ASBR External LSA) packets are created by the ASBR in order to advertise externally redistributed routes into the OSPF's AS. They describe the routes to the destinations external to the OSPF network. These LSAs are forwarded to all the connected and redistributed areas. 

AS External LSA – Type 5

The ASBR generates LSA Type 5 (ASBR External LSA) packets to advertise external redistributed routes into the OSPF's AS. They describe the routes to the destinations external to the OSPF network. These LSAs are forwarded to all the connected and redistributed areas. 

Multicast OSPF LSA – Type 6

Not supported and not used by OSPF. Cisco iOS doesn’t support this LSA type. They use (PIM) Protocol independent Multicast. 

NSSA LSA – Type 7

It is produced by an ASBR inside an NSSA to characterize routes that have been redistributed there. As LSA 7 exits the NSSA, it is converted into LSA 5.

NSSA area routers will be type 7 LSAs, but for other areas (other than NSSA), ABRs will convert them into type 5, which are easily understandable and acceptable in these areas.

These are the 7 OSPF LSA types. Now let's get a hold of another important topic OSPF LSA Header. 

OSPF LSA Header

LINK STATE ADVERTISEMENT (LSA) PACKET STRUCTURE (WITHIN A LINK STATE UPDATE - LSU) 

All the data required to exchange network information inside an OSPF network is contained in the header of each LSA packet. This graphic will allow us to understand what data is contained in an LSA packet. 

The 20-byte LSA header section contains the following fields: 

• LS Age: The number of seconds that have passed since the LSA was created (in seconds), indicating when the first LSA id was created and when it will be promoted. 
• Options: Indicates the OSPF features like tables. 
• LS Type: Defines the LSA types, like where which LSA is being used. 
• Link State ID: Identifies the network link between OSPF routers; mostly, we got to know about the IP Identification used by the OSPF process. 
• Advertising Router: It displays the origin router's ID through which we can identify which process of OSPF is running on which router. 
• LS Sequence Number: A unique numeral to filter out outdated and redundant LS on each LSA packet. 
• LS Checksum: A specific digit supplied to LS for comparison and error detection. 
• Length: Represents LSA packet length, which means the actual size of that LSA is whichever the LSA is being generated. 

This is everything you should know about OSPF LSA types. OSPF is a part of our CCNA and CCNP ENCOR training; for a more detailed explanation, you can check out the CCNA training; for advanced OSPF knowledge, you should check out CCNP ENCOR training. 

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