H3C Technologies H3C SecPath F1000-E - Manual (Page 301)

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NSSA area
Similar to a stub area, an NSSA area imports no AS external LSA (Type-5 LSA) but can import Type-7
LSAs that are generated by the ASBR and distributed throughout the NSSA area. When traveling to the
NSSA ABR, Type-7 LSAs are translated into Type-5 LSAs by the ABR for advertisement to other areas.
In the following figure, the OSPF AS contains three areas: Area 1, Area 2 and Area 0. The other two ASs
employ the RIP protocol. Area 1 is an NSSA area, and the ASBR in it translates RIP routes into Type-7
LSAs and advertises them throughout Area 1. When these LSAs travel to the NSSA ABR, the ABR
translates Type-7 LSAs to Type-5 LSAs for advertisement to Area 0 and Area 2.
On the left of the figure, RIP routes are translated into Type-5 LSAs by the ASBR of Area 2 and distributed
into the OSPF AS. However, Area 1 is an NSSA area, so these Type-5 LSAs cannot travel to Area 1.
Like stub areas, virtual links cannot transit NSSA areas.
Figure 4 NSSA area
Router Types
Classification of Routers
The OSPF routers fall into four types according to their positions in the AS:
1. Internal Router
All interfaces on an internal router belong to one OSPF area.
2. Area Border Router (ABR)
An area border router belongs to more than two areas, one of which must be the backbone area. It
connects the backbone area to a non-backbone area. The connection between an area border router
and the backbone area can be physical or logical.
3. Backbone Router
At least one interface of a backbone router must be attached to the backbone area. Therefore, all ABRs
and internal routers in area 0 are backbone routers.
4. Autonomous System Border Router (ASBR)
A router exchanging routing information with another AS is an ASBR, which may not reside on the
boundary of the AS. It can be an internal router or an area border router.