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.

Figure 4.13. The use of areas reduces link state's demand for system resources.

Figure 4.13. The use of areas reduces link state's demand for system resources.

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