Only a single Universal Transport Zone can exist in a Cross-vCenter topology. As we reviewed in Chapter 2, Configuring VMware NSX Logical Switch Networks, a transport zone is used to define the scope of a logical switch. In the case of a Cross-vCenter NSX environment, the Universal Transport Zone is used to define the scope of universal logical switches and universal logical routers.
As discussed in the explanation in Chapter 2, Configuring VMware NSX Logical Switch Networks, a transport zone defines the scope of where Logical Switches and Distributed Logical Routers can span. In the case of Universal Transport Zone, we define this scope across vCenter boundaries and it usually spans data centers. A vSphere cluster can be connected to a local transport zone as well as a Universal Transport Zone; they are not mutually exclusive.