Data Center Designs

The modern data center has been an evolutionary process. As traffic volumes grew so the did the network. In addition to traffic volumes, other concerns such as security, traffic quality, resiliency, and cost helped to develop first the 3 tier model and later the leaf and spine architectures.

This post is a brief summary of the architectures.

Classic 3 Tier

The classic switched network consists of the following 3 tier hierarchy:

• Core
• Distribution
• Access

The core network originally was a redundant high-speed networking focused almost solely on switching and other functions are in the access and distribution.

The intent of the distribution layer is to provide filtering, quality of service, and potentially routing.

Finally, the access layer was devoted to connecting endpoints such as hosts, servers, access points to the local area network (LAN).

Traffic direction

In the 3 tier model traffic was primarily north/south traffic. Where the clients resided on access switches and the data was on the servers in the data center. Minimal amounts of traffic needed to transit the core to reach other servers.

Leaf / Spine

A leaf and spine network is based on the idea of a Clos network. Which is a multi-stage network traditionally designed for non-blocked telephony fabrics. The concept has been re-purposed with leaf and spine in mind. In the diagram below is a simple instance of leaf and spine.

A data center using leaf and spine would be considered a scale-out architecture. This means that as additional servers are required more leaf nodes can be added to support them.

Traffic direction

As networks and data center evolved more traffic was required between servers. The chokepoint of a core switch became a bottleneck and that facilitated the shift from the 3 tier model towards leaf and spine. In a leaf and spine network, all traffic transits a spine node. This pattern is called east/west traffic.

Wrapping Up

The topic of web-scale companies (e.g. Facebook, Google, Amazon) often comes up when discussing data centers. They use architectures similar to the leaf and spine in which the network is simple (fewer features) but the scale is extremely high.

When reviewing these models in regards to the history of the modern data center it can be helpful to know where we came from and why the data center has evolved to where we are now. Your network is probably NOT a web-scale network. You should consider your requirements when designing your next data center.