OSPF – Areas

Why Areas ?

OSPF, with its multiple databases and complex algorithms, can put greater demands on the memory and processors of a router than other routing protocols. As an internetwork grows, these demands can become significant or even crippling. And although flooding is more efficient than the periodic, full-table updates of RIP and IGRP, it can still place an unacceptable burden on the data links of a large internetwork.

OSPF uses areas to reduce these adverse effects. In the context of OSPF, an area is a logical grouping of  OSPF routers and links that effectively divide an OSPF domain into sub-domains. Routers within an area will have no detailed knowledge of the topology outside of their area.

  • A router must share an identical link state database only with the other routers in its area, not with the entire internetwork. The reduced size of the database reduces the impact on a router’s memory.
  • The smaller link state databases mean fewer LSAs to process and therefore less impact on the
    CPU.
  • Because the link state database must be maintained only within an area, most flooding is also
    limited to the area.

Three types of traffic may be defined in relation to areas:

  • Intra-area traffic consists of packets that are passed between routers within a single area.
  • Inter-area traffic consists of packets that are passed between routers in different areas.
  • External traffic consists of packets that are passed between a router within the OSPF domain and a
    router within another autonomous system.

Backbone Area :  Area ID 0 (or 0.0.0.0) is reserved for the backbone. The backbone is responsible for summarizing the
topographies of each area to every other area. For this reason, all inter-area traffic must pass through the backbone; non-backbone areas cannot exchange packets directly.

Stub Area

A stub area is an area into which AS External LSAs are not flooded. And if type 5 LSAs are not known inside an area, type 4 LSAs are unnecessary; these LSAs are also blocked. ABRs at the edge of a stub area will use Network Summary LSAs to advertise a single default route (destination 0.0.0.0) into the area. Any destination that the Internal Routers cannot match to an intra- or inter-area route will match the default route. Because the default route is carried in type 3 LSAs, it will not be advertised outside of the area.

Restrictions on stub areas
1. As in any area, all routers in a stub area must have identical link state databases. To ensure this condition, all stub routers will set a flag (the E-bit) in their Hello packets to zero; they will not accept any Hello from a router in which the E-bit is set to one. As a result, adjacencies will not be established with any router that is not configured as a stub router.
2. Virtual links cannot be configured within, or transit, a stub area.
3. No router within a stub area can be an ASBR. This restriction is intuitively understandable because ASBRs produce type 5 LSAs and type 5 LSAs cannot exist within a stub area.
4. A stub area may have more than one ABR, but because of the default route, the Internal Routers cannot determine which router is the optimal gateway to the ASBR.
Totally Stubby Areas

If memory is saved by blocking the propagation of type 5 and type 4 LSAs into an area, wouldn’t more memory be saved by blocking type 3 LSAs? In addressing this question, Cisco carries the concept of stub areas to its logical conclusion with a scheme known as totally stubby areas.
Totally stubby areas use a default route to reach not only destinations external to the autonomous system but also all destinations external to the area. The ABR of a totally stubby area will block not only AS External LSAs but also all Summary LSAs—with the exception of a single type 3 LSA to advertise the default route.

Not-So-Stubby Areas
A router with a few stub networks must be attached to the OSPF internetwork via one of the area 2 routers. The router supports only RIP, so the area 2 router will run RIP and redistribute the networks into OSPF. Unfortunately, this configuration makes the area 2 router an ASBR, and therefore area 2 can no longer be a stub area.

The RIP speaker does not need to learn routes from OSPF—a default route pointing to the area 2 router is all it needs. But all OSPF routers must know about the networks attached to the RIP router to route packets to them.

Not-so-stubby areas(NSSAs)[17] allow external routes to be advertised into the OSPF autonomous system while retaining the characteristics of a stub area to the rest of the autonomous system. To do this, the ASBR in an NSSA will originate type 7 LSAs to advertise the external destinations. These NSSA External LSAs are flooded throughout the NSSA but are blocked at the ABR.

The NSSA External LSA has a flag in its header known as the P-bit. The NSSA ASBR has the option of setting or clearing the P-bit. If the NSSA’s ABR receives a type 7 LSA with the P-bit set to one, it will translate the type 7 LSA into a type 5 LSA and flood it throughout the other areas. If the
P-bit is set to zero, no translation will take place and the destination in the type 7 LSA will not be advertised outside of the NSSA.

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