OSPF Special Areas

To understand the OSPF Area types, first let's have a breif review of OSPF LSA types. For deep understanding of LSA types, visit Understanding OSPF Link-state Advertisement.
Routers generate different types of LSAs depending on the router's role within the OSPF Autonomous Systesm.

LSA type 1 (Router LSA)
Generated by all routers in an area to describe their directly attached links (Intra-area routes). These LSAs do not leave the area.

LSA type 2 (Network LSA)
Generated by the DR of a segment to describe the neighbors connected to the segment. These LSAs do not leave the area.

LSA type 3 (Summary LSA)
Generated by ABR to describe a route outside the area (Inter-area routes).

LSA type 4 (ASBR Summary LSA)
Generated by ABR to describe a route to an ASBR to neighbors outside the area.

LSA type 5 (AS External LSA)
Generated by ASBR to describe external routes (routes external to OSPF domain).

LSA type 7 (NSSA External LSA)
Generated by an ASBR inside a NSSA to describe routes redistributed into NSSA. These LSAs are converted back to type 5 LSA by ABR of that area before advertising to other areas.


OSPF Area Types:

• Backbone Area (area 0)
• Normal/Standard Area 
• Stub Area
• Totally Stubby Area (cisco proprietary)
• Not-So-Stubby Area (NSSA)
• NSSA Totally Stubby Area (cisco proprietary)

In a Normal Area, all type of LSAs are allowed. 

►Stub Area is an area in which advertisement of LSA type 4 and LSA type 5 (external routes) are not allowed. Instead a default summary route (0.0.0.0) is inserted into the stub area in order to reach these external routes.
A stub area will have:

• Intra-area routes (LSA type1 and type2)
• Inter-area routes (LSA type3)
• A default route towards ABR  (LSA type3)

For an area to become a stub, all routers belonging to it must be configured to operate as such. Stub routers and non-stub routers will not form adjacencies

Router(config-router)# area xx stub

►Totally Stubby Area is an area in which advertisement of external routes as well as Inter-area routes (LSA type 3, LSA type 4 & LSA type 5) are not allowed. Instead a default summary route (0.0.0.0) is inserted into the stub area in order to reach these external routes.
A Totally Stubby area will have:

• Intra-area routes (LSA type1 and type2)
• A default route towards ABR (LSA type3)

Totally stubby area is Cisco proprietary. For an area to be a Totally Stubby area, the ABR of that area must be a Cisco device. All routers belonging to it must be configured to operate as stub area and the ABR of that area should be configured with no-summary.

Router(config-router)# area xx stub no-summary

►Not-So-Stubby Areas are similar to stubby areas.These areas are shielded from LSA type 5 and LSA type 4 while allowing to import routes from non-OSPF router in the form of type 7 LSA.
By default, a NSSA area will have:

• Intra-area routes (LSA type1 and type2)
• Inter-area routes (LSA type3)
• Redistributed routes (LSA type7)

To configure area as NSSA, use the following command:

Router(config-router)# area xx nssa

Unlike a normal stub area, the ABR will not inject a default route into an NSSA unless explicitly configured to do so. As traffic cannot be routed to external destinations without a default route, you'll probably want to include one by appending default-information-originate.

area xx nssa default-information-originate

►NSSA Totally Stubby Areas are similar to Totally Stubby areas. These areas are shielded from LSA type 3, LSA type 4 and LSA type5 while allowing to import routes from non-OSPF router in the form of type 7 LSA.
By default, a NSSA Totally Stubby Area will have:

• Intra-area routes (LSA type1 and type2)
• Redistributed routes (LSA type7)

To configure an area as a NSSA Totally Stubby Area, use the following command:

Router(config-router)# area xx nssa no-summary

Similar to NSSA, to inject a default route in this area, use the following command:

area xx nssa no-summary default-information-originate