HP 7102dl - ProCurve Secure Router Configuration Manual page 761

Internal routers, which are entirely in one area, handle intra-area routing.
They use Type 1 and 2 LSAs (which are described in "LSA Types" on page
15-37), to synchronize their databases with routers in their own area and to
generate the intra-area routes.
Internal routers forward inter-area traffic using summary routes, which they
generate using the link summaries (Type 3 LSAs) that they receive from their
ABRs. A link summary advertises a connection to a network or range of
networks in another area. The internal router sends the traffic to the non-local
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 15-5 shows several types of OSPF areas.
Area 1
Stub area
Router C
Area 2
Not so
stubby area
Figure 15-5. 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
IP Routing—Configuring RIP, OSPF, BGP, and PBR
Area 0
Network backbone
Router A Router B
Router D
Router E
External
Network
Configuring OSPF
Area 3
Total stub area
Router F
15-35