Areas - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

In a multi-access subnet, such as an Ethernet network, a router can become
a neighbor with all other routers on the subnet. To minimize OSPF packets,
routers elect a DR and a BDR with which all other routers establish full
adjacency. That is, routers send LSAs only to the DR and BDR. Only the DR
broadcasts LSAs. If the DR does not broadcast an LSA in a set amount of
time, the BDR assumes it has failed and takes over as the new DR.

Areas

One of an OSPF network administrator's most important tasks is to group
subnets together into areas so that routers do not need to maintain extensive
and complicated databases to pass traffic smoothly to its destination. An area
is a group of subnets in an OSPF network, each of which runs its own copy of
OSPF and has its own topological database. This means that routers in
separate areas do not need to know each other's topologies or exchange LSAs.
As a result, synchronizing databases consumes less bandwidth. Less powerful
routers and routers that mainly route intra-area traffic no longer have to hold
routing tables that are more extensive than they actually need. (Traffic can
still be routed to other areas through the use of a network backbone, as
explained below.)
Areas can be many different sizes. If possible, however, they should have
contiguous subnets, so that summaries for these subnets can be sent to
other areas.
Areas must be defined so that:
All areas connect to the network backbone, or area 0.
The network backbone consists of the routers that have interfaces on
networks in more than one area, or the ABRs.
The network backbone is contiguous.
Traffic in an OSPF network falls into three categories:
intra-area traffic
inter-area traffic
external traffic
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
13-33), 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
IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring OSPF
13-31