docker创建overlay网络丢失driver？

Question

用这条命令创建一个网络，成功

docker network create -d overlay --subnet=172.16.0.0/24 --gateway=172.16.0.1 cluster

但是具体查看，driver却为空？

[root@xxx]# docker network ls
NETWORK ID          NAME                DRIVER              SCOPE
73aaa2fa0755        bridge              bridge              local
n5f4ajwqlerb        cluster                                 swarm
a09d489d2abd        host                host                local
6xvlx5ivekne        ingress             overlay             swarm
ef2dbca65b2a        none                null                local

[root@xxx]# docker network inspect cluster
[
    {
        "Name": "cluster",
        "Id": "u464ofhqtg3c08cf8ml2zsjfe",
        "Created": "2019-10-14T07:46:10.774583251Z",
        "Scope": "swarm",
        "Driver": "",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "",
            "Options": null,
            "Config": null
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": null,
        "Options": null,
        "Labels": null
    }
]