USG config.gateway.json

{
  "service": {
    "mdns": {
      "repeater": {
        "interface": [
          "eth1.10",
          "eth2.20"
        ]
      }
    }
  },
  "protocols": {
    "igmp-proxy": {
      "interface": {
        "eth1.10": {
          "role": "upstream",
          "threshold": "1"
        },
        "eth2.20": {
          "role": "downstream",
          "threshold": "1"
        },
    "eth0": {
          "role": "disabled"
        },
    "eth1": {
          "role": "disabled"
        },
    "eth2": {
          "role": "disabled"
        },
    "eth2.99": {
          "role": "disabled"
        }
      }
    }
  }
}