Information Technology Reference
In-Depth Information
Area 3
ASBR
Area 0
Internal
Backbone
ABR
NSSA
Stub
Area
Area 2
Area 1
ABR
ASBR
Internal
Figure 11-5
OSPF Stub Networks
To t a l l y S t u b b y A r e a s
Ta ke t he A re a 1 i n Fi g u re 1 1- 5 one step further. The only path for Area 1 to get to Area 0
and other areas is through the ABR. A totally stubby area does not flood network sum-
mary LSAs (Type 3). It stifles Type 4 LSAs, as well. Like regular stub areas, totally stubby
areas do not flood Type 5 LSAs. They send just a single LSA for the default route. If multi-
ple ABRs exist in a totally stubby area, all ABRs inject the default route. Traffic originating
within the totally stubby area routes to the closest ABR.
NSSAs
Notice that Area 2 in Figure 11-5 has an ASBR. If this area is configured as an NSSA, it
generates the external LSAs (Type 7) into the OSPF system while retaining the characteris-
tics of a stub area to the rest of the autonomous system. There are two options for the
ABR. First, the ABR for Area 2 can translate the NSSA external LSAs (Type 7) to au-
tonomous system external LSAs (Type 5) and flood the rest of the internetwork. Second,
the ABR is not configured to convert the NSSA external LSAs to Type 5 external LSAs,
and therefore the NSSA external LSAs remain within the NSSA.
