Description of problem:
IGMP snooping behavior is inconsistent. It is working only if there are subscribed clients and until idle timeout expires. In other cases multicast works as broadcast, i.e. there is flooding to all ports. 
Note: igmp querier is not configured by default.

With the following logical switch configuration igmp snooping works consistently.
other_config        : {mcast_flood_unregistered="false", mcast_snoop="true"}

so workaround is to disable mcast_flood_unregistered for the logical switch:
ovn-nbctl set Logical_Switch <switch ID>  other_config:mcast_flood_unregistered="false"

Version-Release number of selected component (if applicable):
RHOS-16.1-RHEL-8-20201110.n.1

How reproducible:
100%

Steps to Reproduce:
1. Deploy OSP16.1 with NeutronEnableIgmpSnooping set to True
2. Create a network, subnet, security group allowing ssh, icmp, igmp and port for multicast udp traffic (e.g. 5001) 
3. Create 2 VMs connected to the network: sender and unregistered (this one will not subscribe to multicast). If you want to test steps 6-8 create also 2 receiver VMs.
4. On the unregistered VM run tcpdump e.g. tcpdump -i any -vvneA -s0 -l host 225.0.0.1
5. On sender run multicast server script: python3 multicast -s -g 225.0.0.1 -p 5001 -m qweqwe -c 10
Traffic reaches unregistered VM - not as expected.

Also

6. On receivers run multicast client script (see attached file), e.g. python3 multicast -r -g 225.0.0.1 -p 5001
7. On sender run multicast server script (i.e. repeat step 5)
Make sure multicast stream reached both receivers and did not reach the unregistered - as expected
8. Wait at least for 300 seconds until clients subscription expires and repeat step 5
Traffic reaches unregistered VM - not as expected.

Actual results:
Traffic reaches the unregistered VM - BUG

Expected results:
Traffic does not reach unregistered VM even when there are no subscribed clients

Additional info:
[heat-admin@controller-0 ~]$ nb list Logical_Switch
_uuid               : 620f6ceb-23d4-4cd8-b356-6ce614e46f18
acls                : []
dns_records         : []
external_ids        : {"neutron:mtu"="1500", "neutron:network_name"=nova, "neutron:revision_number"="4"}
forwarding_groups   : []
load_balancer       : []
name                : neutron-2cd400a5-7ae4-430c-97d2-67a800d3821c
other_config        : {mcast_flood_unregistered="true", mcast_snoop="true"}
ports               : [3d7b766f-1cd7-4024-8c69-f281c3baff4a, 50dc46c1-5f6b-4381-bf50-239d4c0d58eb, 73a32d4f-7abc-4c11-9fd3-d0c78b4840e4, 8d818d45-ddbe-49bd-8235-abf6c3365635, 9d333c4b-7972-4a31-bf50-628f6c835778, c7cae370-6fb4-4a47-a5b7-5358926402f1]
qos_rules           : []


[heat-admin@controller-0 ~]$ ovn-sbctl list ip_multicast
_uuid               : 3ce47104-3805-4aee-81fa-268b05bbe7d3
datapath            : 01eabea6-a967-41ec-ad6f-32c2a037c063
enabled             : true
eth_src             : ""
idle_timeout        : 300
ip4_src             : ""
ip6_src             : ""
querier             : true
query_interval      : 150
query_max_resp      : 1
seq_no              : 0
table_size          : 2048