OSPF Areas and Area Types

OSPF provides two levels of hierarchy through the use of Areas.  An easy way to explain what an Area is would be:  An OSPF area is a logical segment or a sub-domain containing networks, routers and links sharing the same area id.

Using multiple areas in OSPF reduces the router resource utilization because the routers belonging to the same area only maintain the network topology of the area where they belong. Thus, less LSA has to be flooded and maintained.  In other words, the Routers in other areas; don’t have information about the topology outside its own area.

An area is a 32-bit number which can be represented the same way as an IP address such  “area 0.0.0.0” or as a decimal number, such as “Area 0”.

OSPF-AT-Single-Area

The Area 0 is the backbone area in an OSPF domain.   If more than one area is configured, then Area 0 is required to provide inter-area communications.   This means that in an OSPF autonomous system, all areas must be physically connected to the backbone area 0.

OSPF-AT-Multi-Area

I’ve mentioned that all areas must be connected to Area 0; But, what about areas that cannot be physically connected to the area 0?

OSPF provides a native solution for this problem, OSPF Virtual-Links.  I will dedicate an entire post for this technology later on.

OSPF-AT-Virtual-Link.PNG

The Routers contained within the same area are called Area Routers (AR), the routers with a link in Area Zero (0) and a Non-Zero area is called Area Border Routers (ABRs).  Finally, the Routers with a link in Area Zero (0) or a Non-Zero area and redistributing from another Routing Protocol are called Autonomous System Boundary Routers (ASBRs).   Summarization can be done only on ABRs and ASBRs.

OSPF-Router-Type

OSPF Area Types

The OSPF area types can be divided into the following types:

  • Normal Areas
    • Standard.
    • Transit.
  • Stub Area.
    • Totally Stubby Area.
  • Not-So-Stubby Area (NSSA).
    • Totally NSSA.

Normal Area

Normal Areas can be standard or transit (backbone) areas.

  • The Standard areas are the non-zero defined areas where LSAs Type-1, 2, 3, 4 and 5 are allowed; this means that can accept intra-area, inter-area, and external routes.
  • The backbone area is, in essence, a Standard area but is the area from which all other areas in OSPF must be connected.

I’ve mentioned that summarization is possible only on ABRs and ASBRs; But, what about the default route?    Well I think is necessary to explain about the default-route in normal areas.

It is allowed to generate the default route in Normal Areas.  The default route is injected as Type-5 LSA.

In order to generate the default route use the default-information originate process command.  If the default route is not present in the Routing Table then won’t be generated unless the [always] keyword is added at the end of the command.

OSPF-AREA-Normal

 

Stub Area

Stub areas are typically used where the access to the rest of the network through a single link.  For these type of networks is not necessary having and maintaining a full link state database.   This area type can only accept intra-area, inter-area and the default route (generated by default).

Stub areas allow only LSAs Type-1, 2 and 3.   Redistribution is not allowed in Stub Areas.

The default route in a Stub area is generated by default and is injected in the area as Type-3 LSA.

To configure a Stub Area use the Area [x] stub process command.  This command must be added to the routers belonging to the stub area and also, must be added to the ABR connecting the stub area.

OSPF-AREA-STUB

 

Totally Stubby Area

Totally Stubby areas are an extension of Stub Areas and are in fact, the most restricted type of OSPF area.

In Totally Stubby Areas, the Routers receive only a default route from the ABRs, no External or Summary routes are allowed except for the default-route.   The default route in Totally Stubby area is generated by default and is injected in the area as Type-3 LSA.

To configure a Totally Stubby Area use the Area [x] stub no-summary process command.  This command with the no-summary keyword is required only on the ABR connecting the area.

The router connecting to the ABR does not require the no-summary keyword but requires to be defined as a stub.

OSPF-AREA-T-STUB

Not-So-Stubby Areas (NSSA)

Not-So-Stubby areas are also an extension of the stub areas.  This type of area adds the flexibility of redistribution of external routes into the area while retaining its stub characteristic.   This area type can only accept intra-area, inter-area, external and the default route (which is not generated by default).   Not-So-Stubby areas allow only LSAs Type-1, 2, 3 and 7.

When the default route in an NSSA is generated, it is injected in the area as Type-7 LSA.

To configure an NSSA use the Area [x] nssa [default-information-originate] process command.  This command must be added to the routers belonging to the NSSA area and must be also added to the ABR connecting the NSSA.  The default-information-originate keyword generates the default-route in NSSA.

OSPF-AREA-NSSA

 

Totally Stubby NSSA

Totally Stubby NSSA is an extension to the NSSA.  As a matter of fact, this is the most recommended form of NSSA area type.  This type of area adds the flexibility of redistribution of external routes into the area while retaining it’s totally stubbed characteristic.   This area type can only accept intra-area, external and the default route (which is generated by default).   Totally Stubby NSSA areas allow only LSAs Type-1, 2, [3] and 7.      The default route in a Stub area is generated by default and is injected in the area as Type-3 LSA.

To configure a Totally Stubby NSSA use the Area [x] nssa no-summary process command.  This command with the no-summary keyword is required only on the ABR connecting the area.

The router connecting to the ABR does not require the no-summary keyword but requires to be defined as NSSA.

OSPF-AREA-T-NSSA

It is time to close this post. Thank you for visiting.

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