Installing and Administering Internet Services

Chapter 8 257
Configuring gated
Configuring the OSPF Protocol
Routers that have all their directly-connected networks in the same area
are called internal routers. In Figure 8-3, routers A, B, and H are
internal routers.
Routers that are connected to multiple areas are called area border
routers. In Figure 8-3, routers F and G are area border routers.
Routers that connect one AS to another are called AS boundary
routers. In Figure 8-3, router D is an AS boundary router.
Neighbor routers are routers that interface to a common network.
OSPF uses its own Hello protocol to determine which routers are
neighbors. In Figure 8-3, 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.
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. The Designated Router initiates OSPF protocol
functions on behalf of the network. In Figure 8-3, network 1 can be
accessed through 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 8-3, 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 8-2 shows the various types of link
state advertisements used by the OSPF protocol.