Detecting malicious traffic streams in modern computer networks is a challenging task due to the growing traffic volume that must be analyzed. Traditional anomaly detection systems based on packet inspection face a scalability problem in terms of computational and storage capacity. One solution to this scalability problem is to analyze traffic based on IP flow aggregates. However, IP aggregates can still result in prohibitively large datasets for networks with heavy traffic loads. In this paper, we investigate whether anomaly detection is still possible when traffic is aggregated at a coarser scale. We propose a volumetric analysis methodology that aggregates traffic at the Autonomous System (AS) level. We show that our methodology reduces the number of flows to be analyzed by several orders of magnitude compared with IP flow level analysis, while still detecting traffic anomalies.