2.1. Bandwidth and Traffic Patterns

2.1. Bandwidth and Traffic Patterns

The primary requirement when building an interconnection network for a large number of servers is to accommodate application bandwidth and latency requirements. Until recently it was quite common to see the majority of traffic entering and leaving the data center, commonly referred to as "north-south" traffic. Traditional "tree" topologies were sufficient to accommodate such flows, even with high oversubscription ratios between the layers of the network. If more bandwidth was required, it was added by "scaling up" the network elements, e.g., by upgrading the device's linecards or fabrics or replacing the device with one with higher port density.

Today many large-scale data centers host applications generating significant amounts of server-to-server traffic, which does not egress the DC, commonly referred to as "east-west" traffic. Examples of such applications could be computer clusters such as Hadoop [HADOOP], massive data replication between clusters needed by certain applications, or virtual machine migrations. Scaling traditional tree topologies to match these bandwidth demands becomes either too expensive or impossible due to physical limitations, e.g., port density in a switch.