Overview

OSPFv3 Area Types increase scalability by reducing the demand on network resources. LSA traffic is reduced which reduces bandwidth and CPU usage and they also reduce the size of the Link State Database (LSDB). This database can get quite large!

Area Types

OSPF has five area types:

  • Normal
  • Stub
  • Totally Stubby
  • Not-So-Stubby
  • Totally Not-So-Stubby

The purpose of Area types is to allow, or disallow, LSA types.

Normal

  • Normal areas allow LSA types 1-5:
  • Type 1: Router LSA
  • Type 2: Network LSA
  • Type 3: Inter-Area Prefix LSA
  • Type 4: Inter-Area Router LSA
  • Type 5: AS External LSA

If you need to review Type 1: Router LSA and Type 2: Network LSA, they are explained in the OSPFv3 LSA lesson.

Backbone (Area 0)

The Backbone (Area 0) is a Normal Area.

All OSPF domains must have one, and only one, backbone area. Backbone areas always have an Area ID of 0.

With one exception, all other areas must be directly connected to Area 0. The exception allows for one, and only one, virtual link...a virtual link allows a link that is not physically directly connected to the backbone area to be logically directly connected to the backbone.

Type 3: Inter-Area Prefix LSA

An ABR takes the information received from a Type 1: Router LSA and Type 2: Network LSA, then summarizes them into a Type 3: Inter-Area Prefix LSA.

Routers in another area don’t need to know about the Type 1 and 2 LSAs of another area, but they can still learn the prefixes from another area with the Type 3.

Type 3: Inter-Area Prefix LSAs contains prefixes of another Area.

The Type 3: Inter-Area Prefix LSA is sent to the areas that the ABR is directly connected to. The Type 3 does not exit the areas that receive it.

Normal Area Type 3: Inter-Area Prefix LSA
ABRs summarize Type 1 and 2 LSAs into a Type 3.

  • R-1 sends Type 1 and 2 LSAs to R-2.
  • R-2, because it’s an ABR, summarizes the Type 1 and 2 into a Type 3 and sends it to R-3.
  • R-2 sends Type 1 and 2 LSAs to R-3.
  • R-3, because it’s an ABR, summarizes the Type 1 and 2 into a Type 3 and sends it to R-4.

Type 4: Inter-Area Router LSA and Type 5: AS External LSA

Type 4: Inter-Area Router LSA and Type 5: AS External LSA work together.

Type 4: Inter-Area Router LSA tells other routers how to reach the ASBR to that it can reach the redistributed routes.

Type 5: AS External LSA redistributes external routes into the OSPF domain.

Let's look at Type 4: Inter-Area Router LSA and Type 5: AS External LSA in more detail.

Type 4: Inter-Area Router LSA

If an ABR has an ASBR in its area, it will create and send a Type 4: Inter-Area Router LSA to tell all OSPF domain routers about the ASBR.

A Type 4: Inter-Area Router LSA is used instead of a Type 3: Inter-Area Prefix LSA because some areas block a Type 3: Inter-Area Prefix LSA. A Type 4: Inter-Area Router LSA is accepted by all normal areas.

The Type 4: Inter-Area Router LSA contains the gateway or next hop to the ASBR.

Note the gateway or next hop to the ASBR must be reachable from within OSPF domain.

Type 5: AS External LSA

ASBRs redistribute routes from other Autonomous System (AS) by describing the external prefix and the gateway to reach the external prefix by flooding a Type 5: AS External LSA to connected normal areas.

Note that LSAs 1-3 stay within an area, while 4 and 5 flood to all normal areas. This helps to make OSPF more efficient by reducing LSA traffic and making OSPF’s tables easier to manage.

OSPFv3 Type 4 and Type 5 LSA
Type 4 and 5 LSAs work together to advertise routes for redistributed routes.

  1. R4 ASBR redistributes EIGRP into OSPF by creating and sending a Type 5: AS External LSA into OSPF. In this topology, it is sent to Area 2.
  2. R-3, being an ABR, will see that R4 has set the ASBR bit in the Type 5: AS External LSA denoting the R4 is an ASBR.
  3. R-3 will then summarize the type 5 to a type 4 and send it to all other areas in the OSPF domain.
  4. R-2 passes the Type 4 from R-3 to R-1.

The purpose of having the ABR summarize Type 5 to a Type 4 for the other areas is to remove unnecessary information and thus and improve scalability.

Stub

Stub Areas allow:

  • Type 1: Router LSA and Type 2: Network LSA from within the Stub Area
  • Type 3: Inter-Area Prefix LSA from other areas.

The Stub Area does not receive any other LSA types. Other LSA types (i.e. Type 4: Inter-Area Router LSA and Type 5: AS External LSA) aren’t needed because a stub area simply uses a default route to reach external areas.

Totally Stubby

Totally Stubby Areas are the same as Stub Areas except they don’t allow Type 3: Inter-Area Prefix LSAs in...instead of using a Type 3: Inter-Area Prefix LSA to get to other areas, they use a default route.

Not-So-Stubby Area (NSSA)

The redistribution from another AS into an OSPF Stub Area is not allowed. However, redistribution from another AS is allowed into a Not-So-Stubby Area (NSSA).

Since Type 5: AS External LSA isn’t allowed in an NSSA, ASBRs use a Type 7: AS External LSA.

Note that Type 7: AS External LSA only exist in an NSSA.

When an ABR in the same area as the ASBR receives a Type 7: AS External LSA, it will summarize it into a Type 5: AS External LSA and then send the Type 5: AS External LSA to the other area(s) it’s connected to.

OSPFv3 NSSA
ASBRs redistribute into an NSSA with a Type 7 LSA. ABRs connected to the NSSA summarize the Type 7 into a Type 5 for the other Normal Area(s).

Using the above topology as an example:

  1. Since R4 is an ASBR and its link to Area 2 is configured as an NSSA, it redistributes EIGRP into the OSPF domain by creating a Type 7: AS External LSA and sending it to R-3.
  2. R-3, being an ABR and connected to a Normal area, will summarize the Type 7: AS External LSA into a Type 5: AS External LSA and send it to R-2.

NSSA does not use a default route because it uses a Type 3: Inter-Area Prefix LSA.

Totally Not-So-Stubby Area

A Totally Not-so-stubby is the same as NSSA, but does have a default route.

Route Precedence

OSPF will not always take the shortest path to destination!

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