Data Center Architecture Migration

Posted on by FS.COM

With the increasing expansion of data centers, as well as the development of new technologies such as virtualization, cloud computing, and so on, the network architectures of data centers require to be updated and improved to meet the demands on new computing patterns and applications. Over the past 10 years, the architectures of data centers have been through some changes, from the traditional three-tiered model to leaf-spine. In this post, several architecture models will be presented which help us understand the data center architecture migration better.

Three-Tier Architecture

The three-tier architecture, also called three-level or three-layer architecture, is a traditional data center architecture that consists of Core routers, Aggregation (sometimes called Distribution) routers, and Access switches, as shown in the following picture.

3-tier architecture

It was the primary option of architecture during 2004 to 2007. In a three-tier architecture, the speed of switching layer is 10 Mbps to 100 Mbps. And the speed of SAN (Storage Area Network) is about 2 Gbps to 4 Gbps. Copper cabling is the main cabling option for this architecture. The copper use proportion is about 80%, while fiber is just 20%. Fiber cabling of three-tier architecture includes the use of OM1, OM2, OM3 and SMF.

3-tier architecture fiber-mix

Three-Tier Architecture: End of Row (EoR), Middle of Row (MoR)

End of Row (EoR) and Middle of Row (MoR) architectures are built on three-tier architecture. They are usually used for environments where each row of cabinets is dedicated to a specific purpose and where growth is accomplished on a row-by-row basis. The active device is placed at the end (EoR) or middle (MoR) location to support the servers in that row and structured cabling with patch panels and patch cords serves as the connection point between the switch ports and server ports, as shown in the picture below:


EoR and MoR architectures are considered as new alternatives to the traditional three-tier architecture, during the year of 2007 to 2010. Compared to the traditional three-tier architecture, the proportion of fiber increased has doubled. OM4 has begun to use. Meanwhile, the OM1/OM2 is gradually replaced by OM3/OM4. The speed of switching layer is up to 1000 Mbps, and SAN is up to 8 Gbps.

EoR MoR fiber mix

Three-Tier Architecture: Top of Rack (ToR)

Top of Rack (ToR) is another alternative option based on three-tier architecture. It emerged to the market later than the EoR and MoR (2010 to 2013). In the ToR design, servers connect to one or two Ethernet switches (ToR switches) installed inside the rack. Because all the cabling is inside each rack, the ToR architecture can help minimize the cabling complexity and is more suitable for network migration. Thus, users are more preferred to ToR rather than EoR or MoR.


In a ToR architecture, fiber usage reaches to 60%. OM2 is almost disappeared. OM3 and OM4 have become the primary fiber options for this architecture in data center application. The switch speed has already reached up to 10 Gbps while the SAN has been up to 16 Gbps.

ToR fiber mix

Two-Tier Architecture: Spine and Leaf

Since 2013, there has been a subversive change on data center architecture that data center network design has moved from tree to leaf. And now, the spine-leaf architecture is prevalent in modern data centers. In a spine-leaf architecture, a series of leaf switches form the access layer are fully meshed to a series of spine switches, as shown below.


The spine-leaf architecture is complied with the trend of high-density fiber cabling. Fiber is almost the main media in spine-leaf design (about 80%). Copper is only used in the access layer. OM1 and OM1 are completely replaced by OM3 and OM4 since the switch speed has been 10 Gbps to 40 Gbps and SAN speed has been up to 32 Gbps.

spine-leaf fiber mix


Data center architecture is the physical and logical layout of the resources and equipment within data centers which provides architectural guidelines in data center development. From traditional three-tier to spine-leaf, the trend of data architecture is higher speed, higher density, and more fiber applications. Thus, the step of data center architecture migration will go even further than this. Only thing you can do now is to choose a right and proper architecture for your data center that can meet your needs today—and cost-effectively future-proof your network for tomorrow.

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