In NSX-T Data Center, segments are virtual layer 2 domains. A segment was earlier called a logical switch.

There are two types of segments in NSX-T Data Center:
  • VLAN-backed segments
  • Overlay-backed segments

A VLAN-backed segment is a layer 2 broadcast domain that is implemented as a traditional VLAN in the physical infrastructure. This means that traffic between two VMs on two different hosts but attached to the same VLAN-backed segment is carried over a VLAN between the two hosts. The resulting constraint is that you must provision an appropriate VLAN in the physical infrastructure for those two VMs to communicate at layer 2 over a VLAN-backed segment.

In an overlay-backed segment, traffic between two VMs on different hosts but attached to the same overlay segment have their layer 2 traffic carried by a tunnel between the hosts. NSX-T Data Center instantiates and maintains this IP tunnel without the need for any segment-specific configuration in the physical infrastructure. As a result, the virtual network infrastructure is decoupled from the physical network infrastructure. That is, you can create segments dynamically without any configuration of the physical network infrastructure.

The default number of MAC addresses learned on an overlay-backed segment is 2048. The default MAC limit per segment can be changed through the API field remote_overlay_mac_limit in MacLearningSpec. For more information see the MacSwitchingProfile in the NSX-T Data Center API Guide.