CCNP-Studies
Routing
2-OSPF
6 Ospf Area Types

Area Types (RFC 2328 3)

Backbone area

stub areas:

  1. Stub areas prevent the flooding of Type-5 external LSAs and other external LSA types into them.
  2. In the absence of Type-5 external LSAs, no Type-4 ASBR summary LSAs are generated.
  3. All router links within the area must reach a consensus to define the area as a stub area, signified by the E-Bit setting in their OSPF hello packets.
  4. The ABR's role is to block the propagation of Type-5 LSAs into the stub area.
  5. The ABR distributes a Type-3 default summary route into the area to provide external connectivity to routers within the area.
  6. Stub areas do not support the formation of virtual links.

Totally stubby areas:

  1. They block the flooding of both Type-5 and specific Type-3 LSAs into the area.
  2. Because Type-5 LSAs are absent, Type-4 LSAs are not disseminated either.
  3. These areas are transformed from stub areas by employing the "area x stub no-summary" command on the stub area ABR.
  4. The ABR refrains from injecting Type-3 LSAs for its intra-area and inter-area prefixes into its routing table from other areas. Instead, it exclusively floods a default Type-3 summary LSA into the area.
  5. The backbone area cannot be designated as a totally stubby area.
  6. Virtual links cannot traverse a totally stubby area.

Not-So-Stubby-Areas NSSA:

NSSAs exhibit the following characteristics:

  1. They do the same as stub areas by preventing the propagation of Type-5 external LSAs.
  2. They permit external prefixes to enter the NSSA by distributing them as Type-7 NSSA external LSAs from the NSSA ASBR.
  3. These Type-7 NSSA external LSAs are converted into Type-5 external LSAs by the NSSA ABR before being disseminated to the rest of the OSPF domain.
  4. The backbone area cannot be configured as an NSSA.
  5. Virtual Links cannot traverse through an NSSA.

Totally Not-So-Stubby-Areas NSSA:

Totally NSSAs possess the following characteristics:

  1. They disallow the presence of Type 3 summary and Type-5 external LSAs.
  2. External information is allowed to enter the Area in the form of Type-7 NSSA external LSAs.
  3. The transformation from an NSSA to a totally NSSA is achieved by issuing the "area x nssa no-summary" command on the NSSA ABR.
  4. The NSSA ABR has the authority to determine whether the NSSA becomes a totally NSSA, and this is accomplished by configuring it not to distribute Type-3 summary LSAs into the area.
  5. Once configured with the "area x nssa no-summary" command, the NSSA ABR will disseminate a Type-3 default summary LSA into the area, thereby offering internal routers in the NSSA connectivity to the rest of the domain.
  6. The backbone area cannot be configured as an NSSA.
  7. Virtual Links cannot traverse through an NSSA.
5 Ospf States10 Ospf Commands