HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 646

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring OSPF
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area network to the ABR that advertised it. When this traffic arrives in area 0,
the ABRs route it toward the correct area. When the traffic arrives in the new
area, internal routers use intra-area routing to direct it to its destination.
Autonomous system border routers (ASBRs) support external traffic (in
WANs with one area or with multiple areas.) An ASBR connects to an external
network and runs both OSPF and the external network's routing protocol. It
then injects the external routes, or a default route for external traffic, into the
OSPF network. An ASBR is often in the network backbone, but it can also be
in a stub area that connects to a remote site. When a stub area connects to a
remote site, it is called a not-so-stubby area (NSSA).
Figure 13-5 shows several types of OSPF areas.
Area 1
Stub area
Router C
Area 2
Not so
stubby area
Figure 13-5. OSPF Areas
Stub Areas and Stub Routers. A stub network is a network in which traf-
fic terminates. The network receives traffic destined for its hosts, but it does
not pass any traffic to another network. A stub area is an extension of the idea
of a stub network. A stub area is a group of networks that:
does not forward traffic from one area to another
connects to only one other area—the network backbone or area 0
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.
Area 0
Network backbone
Router A Router B
Router D
Router E
External
Network
Area 3
Total stub area
Router F