OSPF LSA Types

OSPF employs a link state database (LSDB) to store data obtained from link state advertisements (LSAs). OSPF includes several different varieties of LSAs to identify the type of route that is included, and this information is used to determine which routers the LSAs are flooded to. In this topic, we’ll go through the many sorts of LSAs and how they’re flooded, as well as do a lab to demonstrate the impact. To begin, below is a list of LSA variants and their descriptions:

  • LSA Type 1:            Router LSA
  • LSA Type 2:            Network LSA
  • LSA Type 3:            Summary LSA
  • LSA Type 4:            Summary ASBR LSA
  • LSA Type 5:            Autonomous system external LSA
  • LSA Type 6:            Multicast OSPF LSA
  • LSA Type 7:            Not-so-stubby area LSA
  • LSA Type 8:            External attribute LSA for BGP

Here’s the first LSA Type:

Each router within the area will flood a type 1 router LSA within the area. In this LSA you will find a list with all the directly connected links of this router. A link is identified in the LSA using 2 attributes:

  • The IP prefix on an interface.
  • The link type. There are 4 different link types:

Link Type Description Link ID
1 Point-to-point connection to another router. Neighbor router ID
2 Connection to transit network. IP address of DR
3 Connection to stub network. IP Network
4 Virtual Link Neighbor router ID

Note that the router type, or type 1, LSA is flooded only within the same OSPF area.

The network type (type 2) LSA is generated for multi-access/broadcast segments:

For each multi-access network, a network LSA or type 2 LSA is generated. A DR/BDR is required for broadcast and NBMA (non-broadcast multi-access) networks. The DR generates these type 2 LSAs. This LSA contains a list of all routers connected to the multi-access network, the DR, and the multi-access network’s prefix and subnet mask.

The network LSA in the above example will include information about R1, R2, and the DR, as well as the prefix 10.1.2.0/24. The type 2 LSA, like the type 1 LSA, is only flooded within the same OSPF area, it has area scope.

Let’s take a look at the type 3 LSA:

OSPF LSA Type 3
Type 3 LSAs have a domain scope

Type 1 router LSAs never leave the area, they only have area scope. OSPF, on the other hand, operates with many areas, and you will most likely desire complete connection across all of them. R1 is flooding a router LSA in the area, which R2 will record in its LSDB. R3 and R4 must likewise be aware of the networks in Area 69.
R2 will generate a Type 3 summary LSA and flood it into area 0. This LSA will spread over the rest of our OSPF network. As a result, all routers in other regions will be aware of the prefixes from other areas.

The name “summary” LSA is very deceptive. By default, OSPF will not summarize any routing information for you. However, there is a command that allows you to summarize inter-area routes. If you observe several O IA (OSPF inter-area) entries in an OSPF router’s routing table, you’re looking at LSA type 3 summary LSAs. These are your prefixes for other areas!

Time to review the fourth LSA type:

OSPF LSA Type 4

In this example we have R1 that is redistributing information from the RIP router into OSPF. This makes R1 an ASBR (Autonomous System Border Router). What happens is that R1 will flip a bit in the router LSA to identify itself as an ASBR. When R2 who is an ABR receives this router LSA it will R1 in this case is redistributing routing  information from the RIP router into our OSPF domain. R1 is now an ASBR (Autonomous System Border Router). R1 will then toggle a bit (the E bit) in the router LSA to identify itself as an ASBR. When R2, an ABR, sees this router LSA, it generates a type 4 summary ASBR LSA and floods it into area 0. This LSA will be flooded in all other areas as well, as it has domain scope. It is essential so that all OSPF routers know where to look for the ASBR.

OSPF LSA Type 5

Same topology but I’ve added a prefix (1.1.1.0 /24) at our RIP router. This prefix will be redistributed into OSPF. R1 (our ASBR) will take care of this and create a type 5 external LSA for this. Don’t forget we still need type 4 summary ASBR LSA to locate R1. If you ever tried redistribution with OSPF you might have seen O E1 or E2 entries. Those are the external prefixes and our type 5 LSAs.

What about OSPF LSA type 6? Type 6 multicast ospf LSA I can skip because it’s not being used. It’s not even supported by Cisco. We use PIM (Protocol Independent Multicast) for multicast configurations. Type 7 are used for a special area type called NSSA that is only covered in the CCNP ROUTE material.

Let’s check out the Type 5 LSA:

Let’s look at the last LSA type, number 7:

OSPF LSA Type 7

Last LSA type…promised! NSSA areas do not allow type 5 external LSAs. In my picture R1 is still our ASBR redistributing information from RIP into OSPF.

Since type 5 is not allowed we have to think of something else. That’s why we have a type 7 external LSA that carries the exact same information but is not blocked within the NSSA area. R2 will translate this type 7 into a type 5 and flood it into the other areas.

Let me summarize the LSA types for you:

  • Type 1 – Router LSA: The Router LSA is generated by each router for each area it is located. In the link-state ID you will find the originating router’s ID.
  • Type 2 – Network LSA: Network LSAs are generated by the DR. The link-state ID will be the router ID of the DR.
  • Type 3 – Summary LSA: The summary LSA is created by the ABR and flooded into other areas.
  • Type 4 – Summary ASBR LSA: Other routers need to know where to find the ASBR. This is why the ABR will generate a summary ASBR LSA which will include the router ID of the ASBR in the link-state ID field.
  • Type 5 – External LSA: also known as autonomous system external LSA: The external LSAs are generated by the ASBR.
  • Type 6 – Multicast LSA: Not supported and not used.
  • Type 7 – External LSA: also known as not-so-stubby-area (NSSA) LSA: As you can see area 2 is a NSSA (not-so-stubby-area) which doesn’t allow external LSAs (type 5). To overcome this issue we are generating type 7 LSAs instead.

The only thing left to do is take a look at these LSAs in action…time to configure some routers!

This will be coming very soon!

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