IP Multicast: IGMP snooping configuration guide for OVN

How to enable it

In order to enable IGMP snooping with the OVN driver the following configuration needs to be set in the /etc/neutron/neutron.conf file of the controller nodes:

# OVN does reuse the OVS option, therefore the option group is [ovs]
[ovs]
igmp_snooping_enable = True
...

Upon restarting the Neutron service all existing networks (Logical_Switch, in OVN terms) will be updated in OVN to enable or disable IGMP snooping based on the igmp_snooping_enable configuration value.

Note

Currently the OVN driver does not configure IGMP querier in OVN so ovn-controller will not send IGMP group memberships IP querier to retrieve IGMP membership reports from active members.

OVN Database information

The igmp_snooping_enable configuration from Neutron is translated into the mcast_snoop and mcast_flood_unregistered options set in the other_config column from the Logical_Switch table in the OVN Northbound Database:

$ ovn-nbctl list Logical_Switch
_uuid               : d6a2fbcd-aaa4-4b9e-8274-184238d66a15
other_config        : {mcast_flood_unregistered="true", mcast_snoop="true"}
...

To find more information about the learnt IGMP groups by OVN use the command below (populated only when igmp_snooping_enable is True):

$ ovn-sbctl list IGMP_group
_uuid               : 2d6cae4c-bd82-4b31-9c63-2d17cbeadc4e
address             : "225.0.0.120"
chassis             : 34e25681-f73f-43ac-a3a4-7da2a710ecd3
datapath            : eaf0f5cc-a2c8-4c30-8def-2bc1ec9dcabc
ports               : [5eaf9dd5-eae5-4749-ac60-4c1451901c56, 8a69efc5-38c5-48fb-bbab-30f2bf9b8d45]
...

Note

Since IGMP querier is not yet supported in the OVN driver, restarting the ovn-controller service(s) will result in OVN unlearning the IGMP groups and broadcast all the multicast traffic. This behavior can impact when updating/upgrading the OVN services.

Extra information

When multicast IP traffic is sent to a multicast group address which is in the 224.0.0.X range, the multicast traffic will be flooded, even when IGMP snooping is enabled. See the RFC 4541 session 2.1.2:

2) Packets with a destination IP (DIP) address in the 224.0.0.X range
   which are not IGMP must be forwarded on all ports.

The permutations from different configurations are:

  • With IGMP snooping disabled: IP Multicast traffic flooded to all ports.

  • With IGMP snooping enabled and multicast group address not in the 224.0.0.X range: IP Multicast traffic is not flooded.

  • With IGMP snooping enabled and multicast group address is in the 224.0.0.X range: IP Multicast traffic is flooded.