OSPF Area Types

OSPF Area Types
One of our main motivations for subdividing a single OSPF area into multiple areas was to
reduce router overhead. We decided that all routers didn’t need to have the entire network
topology in their link-state databases. Let’s now examine the types of areas that can reduce
router overhead:
Stub area
Whenever a route external to an AS is learned, the ASBR generates a Type 5 LSA and
floods throughout the OSPF autonomous system. Also, an ABR will generate a Type 4 LSA for
ASBRs and flood them throughout an area. In some cases all of the routers in an area don’t need
to know this information. For example, take a look at Figure 6.3.
As you can see in this figure, all routers in Area 1 must send their packets through the ABR in
order for the packets to reach a network external to the AS. In this case, the routers in Area 1
do not need to know about the Type 4 and 5 LSAs, since the ABR handles the forwarding of
packets to networks external to the AS. All the flooding of the Type 4 and 5 LSAs would do is
use up CPU power and memory on the internal routers of Area 1. A better idea would be to
make Area 1 a stub area. That way, the ABR will block the flooding of Type 4 and 5 LSAs, and
instead the ABR will generate a Type 3 LSA with a default route for all networks external to the
AS. The ABR would then flood that and any intra-area Type 3 LSA to all internal routers in
Area 1. Now all of the internal routers for Area 1 would know that if a packet is to be sent to
a network not contained within Area 1, they will forward the packet to the ABR and let it handle
the packet. Important items to keep in mind about stub areas are that if you need to configure
a virtual link, the virtual link cannot traverse a stub area and ASBRs cannot exist in a stub
area. Virtual links is covered in more detail later in this chapter.
Totally stubby area
To further reduce the number of LSAs that an internal router will need to process,
the router can be configured as a totally stubby area. In addition to not propagating Types 4
and 5 LSAs, a totally stubby area does not propagate Type 3 LSAs, except for one Type 3 LSA that
advertises a default route out of the area. The only way an internal router of a totally stubby area
knows how to reach any destination not contained within the local area is through a default route
to the ABR. The function of a totally stubby area is Cisco-specific, which is an important concept to
remember when designing an OSPF network in a multi-vendor routing environment.
Not-so-stubby area (NSSA)
Like a stub area, a not-so-stubby area does not propagate Type 5
LSAs, which means an ASBR cannot be part of a stub area. However, sometimes there is a need,
on a limited basis, to import external routes to an area. Such a situation is where NSSAs, which
will allow an ASBR to participate in the area, are useful. Instead of the ASBR sending out Type 5
LSAs, it will send out Type 7 NSSA External LSAs. The Type 7 LSAs cannot be advertised into
another OSPF area. So what happens is that the ABR for the NSSA receives the Type 7 LSA and
translates it into a Type 5 LSA. The Type 5 LSA is then allowed to be flooded throughout the
OSPF autonomous system.