Create udn in one ns and create testing pods, enable multicast in ns, sending multicast traffic on all pods to join a multicast group. Then disable multicast in ns, and send traffic again, pods can still receive multicast traffic.
If create pods first, and then enable multicast, pods can not receive multicast traffic.

build image via build 4.18.0-0.nightly,openshift/api#2127,openshift/ovn-kubernetes#2403 

oc annotate namespace test k8s.ovn.org/multicast-enabled=true

  % cat udn_l3.yaml 
apiVersion: k8s.ovn.org/v1
kind: UserDefinedNetwork
metadata:
  name: udn-l3 
spec:
  topology: Layer3 
  layer3: 
    role: Primary 
    subnets: 
      - cidr: 10.151.0.0/16
        hostSubnet: 24   

% cat multicast-rc.json 
{
    "apiVersion": "v1",
    "kind": "ReplicationController",
    "metadata": {
	"labels": {
	    "name": "mcast-rc"
	},
	"name": "mcast-rc"
    },
    "spec": {
	"replicas": 3,
	"template": {
	    "metadata": {
		"labels": {
		    "name": "mcast-pods"
		}
	    },
	    "spec": {
		"containers": [
		    {
			"image": "quay.io/openshifttest/mcast-pod@sha256:aa2a8fdcfeddb09097e6a7201cfdb062db8d92d523cbcf758ef7d3545a1f5776",
			"name": "mcast-pod"
		    }
		]
	    }
	}
    }
}  

e.g: % oc rsh mcast-rc-6s2qd 
~ $ omping -c 5 -T 15 10.100.3.3 10.100.2.5 10.100.1.3  

oc annotate namespace test k8s.ovn.org/multicast-enabled- 

10.151.1.3 :   unicast, xmt/rcv/%loss = 10/10/0%, min/avg/max/std-dev = 1.162/1.250/1.348/0.055
10.151.1.3 : multicast, xmt/rcv/%loss = 10/10/0%, min/avg/max/std-dev = 1.214/1.336/1.921/0.209
10.151.5.3 :   unicast, xmt/rcv/%loss = 10/10/0%, min/avg/max/std-dev = 0.454/0.554/0.623/0.046
10.151.5.3 : multicast, xmt/rcv/%loss = 10/10/0%, min/avg/max/std-dev = 0.468/0.568/0.641/0.047