You can implement the physical layer switch fabric of an SDDC by offering Layer 2 or Layer 3 transport services. For a scalable and vendor-neutral data center network, use a Layer 3 transport.

VMware Validated Design supports both Layer 2 and Layer 3 transports. To decide whether to use Layer 2 or Layer 3, consider the following factors:

  • NSX-T service routers establish Layer 3 routing adjacency with the first upstream Layer 3 device to provide equal cost routing for workloads.

  • The investment you have today in your current physical network infrastructure.

  • The benefits and drawbacks for both layer 2 and layer 3 designs.

Benefits and Drawbacks of Layer 2 Transport

A design using Layer 2 transport has these considerations:

  • In a design that uses Layer 2 transport, top of rack switches and upstream Layer 3 devices, such as core switches or routers, form a switched fabric.

  • The upstream Layer 3 devices end each VLAN and provide default gateway functionality.

  • Uplinks from the top of rack switch to the upstream Layer 3 devices are 802.1Q trunks carrying all required VLANs.

Using a Layer 2 transport has the following benefits and drawbacks:

Table 1. Benefits and Drawbacks for Layer 2 Transport

Characteristic

Description

Benefits

  • More design freedom.

  • You can span VLANs across racks.

Drawbacks

  • The size of such a deployment is limited because the fabric elements have to share a limited number of VLANs.

  • You might have to rely on a specialized data center switching fabric product from a single vendor.

  • Traffic between VLANs must traverse to upstream Layer 3 device to be routed.

Figure 1. Example Layer 2 Transport



Benefits and Drawbacks of Layer 3 Transport

A design using Layer 3 transport requires these considerations:

  • Layer 2 connectivity is limited within the data center rack up to the top of rack switches.

  • The top of rack switch ends each VLAN and provides default gateway functionality. The top of rack switch has a switch virtual interface (SVI) for each VLAN.

  • Uplinks from the top of rack switch to the upstream layer are routed point-to-point links. You cannot use VLAN trunking on the uplinks.

  • A dynamic routing protocol, such as BGP, connects the top of rack switches and upstream switches. Each top of rack switch in the rack advertises a small set of prefixes, typically one per VLAN or subnet. In turn, the top of rack switch calculates equal cost paths to the prefixes it receives from other top of rack switches.

Table 2. Benefits and Drawbacks of Layer 3 Transport

Characteristic

Description

Benefits

  • You can select from many Layer 3 capable switch products for the physical switching fabric.

  • You can mix switches from different vendors because of general interoperability between their implementation of BGP.

  • This approach is typically more cost effective because it uses only the basic functionality of the physical switches.

Drawbacks

  • VLANs are restricted to a single rack. The restriction can affect vSphere Fault Tolerance, and storage networks.

Figure 2. Example Layer 3 Transport