HP 7102dl - ProCurve Secure Router Configuration Manual page 762

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring OSPF
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Internal routers in a stub area are stub routers. At least one router in the area
communicates with an ABR in area 0. The network that the two routers have
in common is defined as part of the stub area, making the area 0 router part
of both area 0 and the stub area. This topology prevents routers from process-
ing superfluous information.
Routers in the stub area deal primarily with intra-area LSAs. The ABR sum-
marizes routes to the area and sends these routes to other ABRs in the
backbone to support inter-area routing. The ABR also sends summary routes
(Type 3 LSAs) for other areas into the stub area.
Stub routers do not receive Type 5 LSAs, which are for external routes. They
can, however, receive a default route from their ABR that allows them to route
traffic to any external destination. Because external routes are often responsi-
ble for the bulk of LSAs and routing table entries, this policy greatly relieves the
strain on stub routers.
You can also configure a stub area to not receive summary routes from its
ABR. Such an area is sometimes called a total stub area. It receives only a
default route for forwarding all inter-area and external traffic.
Backbone (Area 0). A network's backbone, or area 0, links all stub areas.
As discussed above, it consists of the ABRs. Through exchanges with other
ABRs in the backbone, all ABRs hold a topological database for the entire
network. They generate route summaries for each non-backbone area. They
then send these route summaries to each other and to the internal routers they
service. Obviously, ABRs must process more routes than stub routers and
need correspondingly more power.
Area 0 might also contain ASBRs, which communicate with routers external
to the AS (for example, an ISP) and redistribute external routes into the
OSPF network.
Area 0 can also contain internal routers, which, like internal routers in a stub
area, route intra-area traffic. For example, an organization's headquarters
might be its area 0. This area would contain the ABRs that communicate with
remote sites and also routers that support only the headquarters LAN.
NSSA. An NSSA is an area that resembles a stub area in most ways. It
connects to the network backbone and typically does not pass traffic to other
areas. However, a router in an NSSA also connects to a remote site or an ISP
through an ASBR. Typically, OSPF would not permit the external routes to be
distributed into the stub area. However, internal routers in an NSSA can
receive specially defined LSAs for external routes.