NOTE
Potential shortcomings of link state routing
An area is a subset of the routers that make up an internetwork. Dividing an internetwork into areas is a
response to three concerns commonly expressed about link state protocols:
The necessary databases require more memory than a distance vector protocol requires.
The complex algorithm requires more CPU time than a distance vector protocol requires.
The flooding of link state packets adversely affects available bandwidth, particularly in unstable
internetworks.
Modern link state protocols and the routers that run them are designed to reduce these effects, but cannot
eliminate them. The last section examined what the link state database might look like, and how an SPF
algorithm might work, for a small eight-router internetwork. Remember that the stub networks that would
be connected to those eight routers and that would form the leaves of the SPF tree were not even taken
into consideration. Now imagine an 8000-router internetwork, and you can understand the concern about
the impact on memory, CPU, and bandwidth.
This impact can be greatly reduced by the use of areas, as in Figure 4.13. When an internetwork is
subdivided into areas, the routers within an area need to flood LSAs only within that area and therefore
need to maintain a link state database only for that area. The smaller database means less required
memory in each router and fewer CPU cycles to run the SPF algorithm on that database. If frequent
topology changes occur, the resulting flooding will be confined to the area of the instability.
The routers connecting two areas (Area Border Routers, in OSPF terminology) belong to both areas and
must maintain separate topological databases for each. Just as a host on one network that wants to send a
packet to another network only needs to know how to find its local router, a router in one area that wants
to send a packet to another area only needs to know how to find its local Area Border Router. In other
words, the intra-area router/inter-area router relationship is the same as the host/router relationship but at
a higher hierarchical level.
Distance vector protocols, such as RIP and IGRP, do not use areas. Given that these protocols have no
recourse but to see a large internetwork as a single entity, must calculate a route to every network, and
must broadcast the resulting huge route table every 30 or 90 seconds, it becomes clear that link state
protocols utilizing areas can actually save system resources.