HP-UX Routing Services Administrator's Guide HP-UX 11i v2, HP-UX 11i v3 (B2355-91153, November 2011)

Figure 6 Areas Defined in an Autonomous System
Internal routers have all their directly connected networks in the same area. In Figure 6, routers A,
B, and H are internal routers.
Routers that are connected to multiple areas are called area border routers. In Figure 6, routers F
and G are area border routers.
Routers that connect one AS to another are called AS boundary routers. In Figure 6, router D is
an AS boundary router.
Neighbor routers are routers that interface to a common network. OSPF uses its own Hello
subprotocol to determine which routers are neighbors. In Figure 6, routers A, B, and C are a set
of neighbor routers that interface to network 1, while routers A and F are another set of neighbor
routers that interface to network 2.
NOTE: The Hello subprotocol used with OSPF is not the same as the gated HELLO protocol.
The Hello subprotocol is still supported.
Multi-access networks (networks that can be accessed through two or more neighbor routers) must
have one of the routers identified as a designated router.
Designated routers initiate OSPF protocol functions on behalf of the network. In Figure 6, you can
access network 1 through the neighbor routers A, B, or C; one of these routers is elected to become
the designated router for network 1.
The set of routers that exchange OSPF protocol packets between areas in an autonomous system
is called the backbone. In Figure 6, routers C, D, E, F, G, and I form an AS backbone that allows
protocol packets to travel between the three areas.
OSPF routers exchange various types of link state advertisements to build their topological databases.
Most link state advertisements are flooded (sent to every router) throughout the attached area. An
exception is the link state advertisement sent out by AS boundary routers that describe routes to
destinations outside the AS; these advertisements are flooded throughout the AS. Table 2 shows
the various types of link state advertisements used by the OSPF protocol.
28 Configuring gated