TMS zl Management and Configuration Guide ST.1.1.100430

9-29
Routing
OSPF
OSPF defines specific rules for synchronizing databases with a minimum of
traffic between routers. Any two routers running OSPF on the same interface
are neighbors that could potentially send each other LSAs. However, not all
neighbors establish full adjacency—that is, exchange LSAs. OSPF institutes
protocols by which all routers can synchronize their databases without all of
them exchanging LSAs.
Areas
When using OSPF, one of the 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 9-32),
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 traffic that is destined to