HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 651

One common topology for a WAN is a headquarters, defined as area 0, that
connects to stub areas at one or more remote sites. In this topology, the
headquarters' routers that connect to the remote sites are ABRs. The routers
at the remote sites are internal routers. If a router connects to a public or other
external network, such as an ISP, it is an ASBR. (See Figure 13-6.)
Network 4
Site A
Figure 13-6. OSPF Network with Headquarters (HQ) as Area 0
Another topology for an OSPF WAN is a network backbone, consisting of the
network on which each router's WAN interface resides, and a series of areas,
consisting of the LAN or LANs at each site. In other words, routers or routing
switches in each LAN would be the internal routers for that stub area. The
ProCurve Secure Routers that provide WAN connections to the remote sites
would be ABRs. These routers would have one or more interfaces in area 0
and one or more interfaces in the local stub area. They would thus string stub
areas together. Such a topology is best suited for two or more remote sites of
roughly equal complexity. (See Figure 13-7. Note that this figure simplifies the
topology. In reality, each stub area would have many more routers or routing
switches than the ones shown in the figure.)
IP Routing—Configuring RIP, OSPF, BGP, and PBR
HQ Area 0
Internal
Router A
Network 1
Internal ABR B
Router C
Stub
area 1
Internal
Router D
Stub
area 2
Configuring OSPF
Network 2
Network 3
Network 5
Site B
13-37