Since bandwidth, RAM, and CPU cost money (and we, unfortunately, don’t have infinite money), there is a limit to how large an OSPF autonomous system can grow before you run into problems.
There are two solutions to this scalability problem, you either limit OSPF to small internetworks (bad idea!) or modify OSPF to enhance its scalability (good idea!). When using option 2 (called “multi-area OSPF”), we divide the OSPF autonomous system into regions called areas.
The goal of summarization is to conserve RAM, bandwidth and CPU by hiding information from some of the routers, but as a result of hiding information, routing may no longer be optimal. Note that this is not just a characteristic of OSPF, it applies to any routing protocol when there are multiple paths available and summarization is being used. That being said, summarization techniques are still tremendously useful, as long as they’re not applied blindly.
Continue reading to learn more about OSPF scaling and summarization.