The functionality in an SDDC is distributed across multiple workload domains and vSphere clusters. A workload domain, whether it is a management domain or virtual infrastructure workload domain, is a logical abstraction of compute, storage, and network cloud capacity and consists of one or more clusters. Each cluster can exist vertically in a single rack or be spanned horizontally across multiple racks. You determine the total number of racks for each cluster type according to your scalability needs.

The relationship between workload domains and data center racks in VMware Cloud Foundation is not one-to-one. While a workload domain is an atomic unit of repeatable building blocks, a rack is a unit of size. Because workload domains can have different sizes, you map workload domains to data center racks according to your requirements and physical infrastructure constraints.

When using a Layer 3 network fabric, the clusters in the management domain cannot span racks. Management appliances and virtual machines rely on VLAN-backed networks. The physical network configuration terminates Layer 2 networks in each rack at the top-of-rack (ToR) switch. Therefore, you cannot migrate a virtual machine to a different rack because the IP subnet is available only in the rack where the virtual machine is currently running. However, you can map each management cluster to a single rack.

Table 1. Management Domain to Rack Configuration Options

Management Domain to Rack Configuration

Description

Management domain in one rack

The management domain can occupy exactly one rack, whether it consists of a single or multiple clusters.

Management domain across multiple racks

To span multiple racks, the network fabric must support stretched Layer 2 networks between these racks. Otherwise, map each cluster in the management domain to a single rack.

Figure 1. Management Domain in One Rack

One rack runs the default cluster of the management domain.
Figure 2. Management Domain Spanning Multiple Racks


Figure 3. Management Domain with Multiple Availability Zones in One Rack


Figure 4. Management Domain with Multiple Availability Zones Spanning Multiple Racks
In an environment with multiple VMware Cloud Foundation instances, for example, you can have two availability zones in the first instance only. Each zone spans over the default management cluster on a single rack server.