Review the NSX-V topology and decide how to map it to the NSX-T topology. During the migration, the "Define a Topology" step will prompt you for the mapping.

If you are running NSX-T 3.2.0 or 3.2.1, you can migrate an NSX-V load balancer to NSX-T Advanced Load Balancer (ALB). Starting with NSX-T 3.2.2, you can only migrate an NSX-V load balancer to an NSX-T load balancer.

Specifically, determine how many NSX-T Edge clusters you need and how Edge Service Gateways (ESGs) and Distributed Logical Routers (DLRs) should map to gateways in NSX-T. The northbound ESGs without any L4-L7 services should be skipped. These are usually the ESGs peering with northbound routers and are in ECMP path. If you are using VPN on a northbound ESG, migrating to active-standby tier-0 is recommended. In other cases, migrating the ESGs/DLRs to tier-1 is recommended. An ESG and a DLR can be merged in one mapping entry in the mapping file.

You cannot map multiple tier-1 gateways without an edge cluster or a DR-only tier-1 gateway under a parent tier-0 gateway to DLRs. You also cannot map the parent tier-0 gateway to a DLR if you are mapping to a DR-only tier-1 gateway. If your topology requires mapping multiple DLRs, you must use an active-standby tier-1 gateway with an edge cluster assigned.

If you have a DHCP server configured on an ESG and DHCP relay server configured on a DLR, you must map the ESG and DLR to the same NSX-T gateway.

An example of a mapping file that maps ESGs to a tier-0 gateway: 
[
  {
    "name":"nsxv-to-nsxt-mapping",
    "v_edges_to_policy_gateways_mappings":[
      {
        "v_edges":[
          "edge-1",
          "edge-2"
         ],
         "policy_gateway_name": "tier0-gateway"
         "policy_gateway_path": "/infra/tier-0s/tier0-gateway"
      }
    ]
  }
]
If you are running NSX-T 3.2.0 or 3.2.1, and you are doing a configuration migration, the above-mentioned mapping is not used for Advanced Load Balancer (ALB). Instead there is another optional mapping that you may provide for ALB. To specify that mapping, you must upload a JSON file. An example of a mapping file that maps ESGs to Service Engine groups: 
{
  "alb": {
    "service_engine_group_per_esg": false,
    "esgs": [
      {
        "name": "edge-4",
        "interfaces": [
          {
            "name": "mgmt",
            "tier1_id": "London_Tier1Gateway1"
          },
          {
            "name": "vnic1",
            "placement_network_subnet": "172.16.1.10/16",
            "service_engine_group": "Test-SE-group"
          }
        ]
      }
    ]
  }
}

For more information about creating a mapping file for the load balancer, see Migrating NSX-V Load Balancer to Advanced Load Balancer.

Starting with NSX-T 3.2.1, you can migratie a cross-vCenter environment to NSX Federation. Here is a sample mapping file for such a migration: 
[
  {
    "name": "site-GM",
    "nsxv_id": "20.20.0.131",
    "nsxt_site_id": "",
    "v_edges_to_policy_gateways_mappings": [
      {
        "v_edges": [
          "edge-3",                                          <== Dhcp server
          "edge-1b4b70c4-27da-4ee9-91cb-bd6389dadca2"        <== Dhcp Relay
        ],
        "policy_gateway_name": "tier0_1_global",
        "policy_gateway_path": "/global-infra/tier-0s/tier0_1_global"
      },
      {
        "v_edges": [
          "edge-a529c168-56c0-4e1a-98e4-e1f0312d82a4"
        ],
        "policy_gateway_name": "tier1_0_global",
        "policy_gateway_path": "/global-infra/tier-1s/tier1_0_global".     <== mapping corresponding to UDLR
      },
      {
        "v_edges": [
          "edge-5"
        ],
        "policy_gateway_name": "tier0_2_global",
        "policy_gateway_path": "/global-infra/tier-0s/tier0_2_global"
      }
    ]
  },
  {
    "name": "london",
    "nsxv_id": "20.20.0.131",
    "nsxt_site_id": "23790eb0-201a-48c3-8e34-8a03be03b61a",
    "v_edges_to_policy_gateways_mappings": [
      {
        "v_edges": [
          "edge-4"
        ],
        "policy_gateway_name": "site0_tier1_0",
        "policy_gateway_path": "/infra/tier-1s/site0_tier1_0"
      }
    ]
  },
  {
    "name": "paris",
    "nsxv_id": "20.20.0.132",
    "nsxt_site_id": "27db83d3-aa10-4a40-b15c-bf6e234b9e74",
    "v_edges_to_policy_gateways_mappings": [
      {
        "v_edges": [
          "edge-1",
          "edge-5"
        ],
        "policy_gateway_name": "site1_tier0_1_local",
        "policy_gateway_path": "/infra/tier-0s/site1_tier0_1_local"
      }
    ]
  }
]
If you are migrating a cross-vCenter environment to NSX Federation, note the following:
  • Tier-0 gateways created on Local Manager must have an Edge cluster assigned.
  • Tier-1 gateways created on Local Manager must either have an Edge cluster assigned or be connected to a tier-0 gateway that has an Edge cluster assigned.
  • Tier-0 and tier-1 gateways created on Global Manager must span all the sites.
  • In the mapping file, when specifying attributes for the Global Manager, nsxt_site_id must be an empty string.
  • Universal DLR (UDLR) should not be merged with any other ESG.
  • Northbound ESGs with L4-L7 services peering with UDLR must be mapped to a local site's active-standby tier-0 gateway.