macvlan why "master" field is required

[ccmin]

Accroding to macvlan README, the master is optional,as follows:
master (string, optional): name of the host interface to enslave. Defaults to default route interface.
But when i delete the "master" in the yaml

[root@master ~]# cat macvlan-conf-1.yaml
apiVersion: "k8s.cni.cncf.io/v1"
kind: NetworkAttachmentDefinition
metadata:
name: macvlan-conf-1
spec:
config: '{
"cniVersion": "0.3.0",
"type": "macvlan",
"mode": "bridge",
"ipam": {
"ranges": [
[ {
"subnet": "192.168.2.0/24",
"rangeStart": "192.168.2.82",
"rangeEnd": "192.168.2.86",
"gateway": "192.168.2.4"
} ]
]
}
}'
[root@master ~]#
Then i apply the pod, it'state is still ContainerCreating, and describe information is
Events:
Type Reason Age From Message
Normal Scheduled 7s default-scheduler Successfully assigned default/pod-case-02 to master
Warning FailedCreatePodSandBox 0s kubelet, master Failed create pod sandbox: rpc error: code = Unknown desc = [failed to set up sandbox container "5eef8919a1112abe8fdd108711c2629d77051cdf874ac714beea565c16774160" network for pod "pod-case-02": NetworkPlugin cni failed to set up pod "pod-case-02_default" network: Multus: Err adding pod to network "macvlan-conf-1": Multus: error in invoke Delegate add - "macvlan": "master" field is required. It specifies the host interface name to virtualize, failed to clean up sandbox container "5eef8919a1112abe8fdd108711c2629d77051cdf874ac714beea565c16774160" network for pod "pod-case-02": NetworkPlugin cni failed to teardown pod "pod-case-02_default" network: Multus: error in invoke Delegate del - "macvlan": "master" field is required. It specifies the host interface name to virtualize / Multus: error in invoke Conflist Del - "cbr0": error in getting result from DelNetworkList: failed to convert major version part "": strconv.Atoi: parsing "": invalid syntax]
Normal SandboxChanged 0s kubelet, master Pod sandbox changed, it will be killed and re-created.
i understanding the "master" should use the default route interface ens2f2
[root@master ~]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.1.1 0.0.0.0 UG 100 0 0 ens2f2