3Com 4007, Implementation Manual, Page: 451 / 684

OSPF Overview 451
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Ability to partition the network into more manageable areas —
Many autonomous systems in the Internet are large and complicated
to manage. OSPF allows them to be subdivided into smaller, more
manageable networks or sets of contiguous networks called
areas .
You can think of an area as a generalization of an IP subnetworked
network. The topology of an area is hidden from the rest of the AS,
which significantly reduces routing traffic and also serves to lend the
area protection from bad routing data. By partitioning the network
into areas, OSPF limits the topology map required in each router. This
limitation in turn conserves processing and memory requirements in
each router, as well as reduces the amount of link state information
being flooded onto the network.
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Authentication for protocol exchanges — All OSPF protocol
exchanges are authenticated, which means that only known, trusted
routers can participate in routing updates. OSPF supports a variety of
authentication schemes, with a single scheme configured for each
area. This partitioning allows some areas to use much stricter
authentication than others.
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Host-specific and network-specific route support — OSPF
supports traffic forwarding to single hosts or networks. Each network
the router knows has both an IP destination address and a mask. The
mask indicates the number of nodes on the network. A mask of all
ones (0xffffffff) indicates a presence of a single node on the network
(called a
stub network ).
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Support for designated and back-up designated routers — OSPF
works by exchanging information between adjacent routers, not
neighboring routers. To avoid the need for every router on a LAN or
area to talk to every other router on a multiaccess network (a network
that has at least two attached routers), one router is elected as the
designated router. The designated router is considered adjacent to all
other routers in the area and exchanges information with them.
Routers that are not adjacent to each other do not exchange
information. Therefore, instead of all routers on the network sending
Link State Advertisements (LSAs), only the designated router sends
LSAs. This feature significantly reduces data and routing traffic.