Finally, the link-state database, which must contain a complete
topology of the network, will also threaten to consume resources and
bog down the router.
Fortunately, OSPF allows large areas to be separated into
smaller, more manageable areas that can exchange summaries of
routing information rather than exchange every detail. By
splitting the network into manageable pieces, OSPF routers can scale
gracefully.
Just how many routers can an OSPF area support? Field studies
have shown that a single OSPF area should not stretch beyond 50
routers, although there is no concrete limit. Some areas may do fine
with more than 50 routers. Other areas, particularly those with
unstable links, may need to operate with fewer than 50 routers.
Ultimately, you must determine just how many routers a particular
OSPF area can handle. Knowing your network, by tracking performance
and monitoring usage, is the only way to accurately gauge whether an
OSPF area can support 20, 30, or 60 routers.
OSPF's capability to separate a large internetwork into multiple
areas is referred to as hierarchical routing. Hierarchical routing
enables you to separate large internetworks into smaller
internetworks that are called areas. With this technique, interarea
routing still occurs. Interarea routing is the process of exchanging
routing information between OSPF areas. However, interarea routing allows
OSPF to summarize and contain area-specific information so that many
of the smaller internal routing operations, such as recalculating
the database, are restricted within an area.
For example, if Area 1, shown in Figure
,
is having problems with a link going up and down (flapping), routers
in other areas do not need to run their Shortest Path First (SPF)
calculation because they are isolated from the problems in Area 1.
The hierarchical topology possibilities of OSPF have several
important advantages:
- Reduced frequency of SPF calculations - Because
detailed route information is kept within each area, it is not
necessary to flood all link-state changes to all other areas.
Thus, only those routers affected by a change need to run the
SPF calculation.
- Smaller routing tables - When using multiple areas,
detailed route entries for specific networks within an area are
kept inside the area. Rather than advertise these explicit
routes outside the area, you can have the routes summarized into
one or more summary routes. Advertising these summaries reduces
the amount of LSAs propagated between areas but allows all
networks to remain reachable.
- Reduced link-state update (LSU) overhead - LSUs can
contain a variety of LSA types, including link-state information
and summary information. Rather than send an LSU about each
network to every area, you can advertise a single route or a few
summarized routes between areas to reduce the overhead
associated with LSUs that cross multiple areas.
Hierarchical routing increases routing efficiency because it
allows you to control the type of routing information that flows
into and out of an area. OSPF provides for different types of routing
updates, depending on the type of area and the number of areas that
a router connects to. The following sections describe the different
roles that an OSPF router can play, the types of LSAs that it can
use, and the types of areas that it can connect to.