There are two main disadvantages of route aggregation :
* Suboptimal routing. Misconfigured route summarization may result in suboptimal routing. Route summarization may also create inconsistent routing if a network has noncontiguous subnetworks. When using summaries, the router may prefer another path where it has learned a more specific network form, which may not be the most optimal routing method.
* Forwarding traffic for unused networks. If the router doesn't find a matching destination in its routing table, it will start dropping traffic, leading to data loss. Also, the summary route may cover unused networks. The router that has a summary route will forward traffic to the router that advertised the summary route.
To avoid suboptimal or incorrect routing and to prevent routers from inaccurately advertising networks or duplicating other routers' advertisements, it's important to design networks with summarization in mind. Advance planning and leaving room for future network growth can help with the design of a scalable network that supports route summarization.
