OSPF Area Types

What is OSPF Area?

OSPF allows collections of contiguous networks and hosts to be grouped together.  Such a group, together with the routers having interfaces to any one of the included networks, is called an area. (RFC 2823)

OSPF areas are logical groupings of routers and networks devices. Each area contains separate LSD (link state database) whose information may be summarized towards the rest of the network by the connecting router. In OSPF area is represented by 32 bit area identifiers. The topology of an area is unknown outside of the area. The advantages of an area is include

·         Less routing Traffic in an area

·         Less Router’s memory consumption

·         Less CPU utilization

·         Smaller routing tables

OSPF area Types

      Standard Area

      Backbone Area

      OSPF Stub area

      OSPF Totally stubby Area

      OSPF Not So stubby area

Standard Area

Normal area is known as Standard areas. This is basic types of area in OSPF. When you define an OSPF area with default setting e.g. which have no stub, backbone etc. is simply referred as standard area.

A standard area can contain LSAs of type 1,2,3,4, and 5.

Backbone Area

In OSPF area 0 is backbone area. A backbone area contains all the information of its connected areas. Backbone area is also responsible for distributing routing information to other area and to other automatous system connected to OSPF. All OSPF areas are must be connected to backbone area. This is basic requirement in OSPF, If this is not implemented then you need Virtual Links.

OSPF Stub area:

Stub area are good feature of OSPF, by using this you can minimize routing updates for a specific area and can make routing table smaller. OSPF stub Area block the external area route, block type-5 LSA for some area.

OSPF Stub area configuration:

For example in figure we want to make area-2 as stub so that R1 don’t receive any update of other areas but just a default route to R2. Following commands accomplish this goal. 

R1(config)#router ospf 1

R1(config-rtr)#area 2 stub

R2(config)#router ospf 1

R2(config-rtr)#area 2 stub

Read more for OSPF Stub Area Configuration

Totally stubby Area:

Totally stubby area is cisco proprietary and it blocks type 3, 4 and 5 LSA from entering into an area. Configurations are simple, you can see below.

R4(config-rtr)#area 1 stub no summary

R5(config-rtr)#area 1 stub

OSPF Not So stubby area:

NSSA is define in RFC 1587. A not-so-stubby area (NSSA) is a type of stub area that can import autonomous system external routes and send them to other areas. Since Type 5 LSAs are not allowed in NSSA areas, so the ASBR in NSSA generates a type 7 LSA instead, this type-7 LSA passes to backbone area and then converted back to types-5 LSA.

For more detail you can visit OSPF NSSA configuration.