IGMP (Internet Group Management Protocol) and MLD (Multicast Listener Discovery) snooping are implemented in the bridge driver in the Cumulus Linux kernel and are enabled by default. IGMP snooping processes IGMP v1/v2/v3 reports received on a bridge port in a bridge to identify the hosts which would like to receive multicast traffic destined to that group.

When an IGMPv2 leave message is received, a group specific query is sent to identify if there are any other hosts interested in that group, before the group is deleted.

An IGMP query message received on a port is used to identify the port that is connected to a router and is interested in receiving multicast traffic.

MLD snooping processes MLD v1/v2 reports, queries and v1 done messages for IPv6 groups. If IGMP or MLD snooping is disabled, multicast traffic gets flooded to all the bridge ports in the bridge.  Similarly, in the absence of receivers in a VLAN, multicast traffic would be flooded to all ports in the VLAN. The multicast group IP address is mapped to a multicast MAC address and a forwarding entry is created with a list of ports interested in receiving multicast traffic destined to that group.

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Configuring IGMP/MLD Querier

If no multicast router is sending queries to configure IGMP/MLD querier on the switch, you can add a configuration similar to the following in /etc/network/interfaces. To enable IGMP and MLD snooping for a bridge, set bridge-mcquerier to 1 in the bridge stanza. By default, the source IP address of IGMP queries is 0.0.0.0. To set the source IP address of the queries to be the bridge IP address, configure bridge-mcqifaddr 1.

For an explanation of the relevant parameters, see the ifupdown-addons-interfaces man page.

For a VLAN-aware bridge, use a configuration like the following:

auto bridge.100
vlan bridge.100
  bridge-igmp-querier-src 123.1.1.1
 
auto bridge
iface bridge
  bridge-ports swp1 swp2 swp3
  bridge-vlan-aware yes
  bridge-vids 100 200
  bridge-pvid 1
  bridge-mcquerier 1

For a VLAN-aware bridge, like bridge in the above example, to enable querier functionality for VLAN 100 in the bridge, set bridge-mcquerier to 1 in the bridge stanza and set bridge-igmp-querier-src to 123.1.1.1 in the bridge.100 stanza.

You can specify a range of VLANs as well. For example:

auto bridge.[1-200]
vlan bridge.[1-200]
  bridge-igmp-querier-src 123.1.1.1

For a bridge in traditional mode, use a configuration like the following:

auto br0
iface br0
  address 192.0.2.10/24
  bridge-ports swp1 swp2 swp3
  bridge-vlan-aware no
  bridge-mcquerier 1
  bridge-mcqifaddr 1

Disable IGMP and MLD Snooping

To disable IGMP and MLD snooping, set the bridge-mcsnoop value to 0.

Example Disable IGMP MLD Snooping Configuration

The example NCLU commands below create a VLAN-aware bridge interface for a VRR-enabled network:

cumulus@switch:~$ net add bridge bridge mcsnoop no
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

The commands above add the bridge-mcsnoop line to the following example bridge in /etc/network/interfaces:

auto bridge
iface bridge
  bridge-mcquerier 1
  bridge-mcsnoop 0
  bridge-ports swp1 swp2 swp3
  bridge-pvid 1
  bridge-vids 100 200
  bridge-vlan-aware yes

Debugging IGMP/MLD Snooping

To get the IGMP/MLD snooping bridge state, run brctl showstp <bridge>:

cumulus@switch:~$ sudo brctl showstp bridge
 bridge
 bridge id              8000.7072cf8c272c
 designated root        8000.7072cf8c272c
 root port                 0                    path cost                  0
 max age                  20.00                 bridge max age            20.00
 hello time                2.00                 bridge hello time          2.00
 forward delay            15.00                 bridge forward delay      15.00
 ageing time             300.00
 hello timer               0.00                 tcn timer                  0.00
 topology change timer     0.00                 gc timer                 263.70
 hash elasticity        4096                    hash max                4096
 mc last member count      2                    mc init query count        2
 mc router                 1                    mc snooping                1
 mc last member timer      1.00                 mc membership timer      260.00
 mc querier timer        255.00                 mc query interval        125.00
 mc response interval     10.00                 mc init query interval    31.25
 mc querier                0                    mc query ifaddr            0
 flags

swp1 (1)
 port id                8001                    state                forwarding
 designated root        8000.7072cf8c272c       path cost                  2
 designated bridge      8000.7072cf8c272c       message age timer          0.00
 designated port        8001                    forward delay timer        0.00
 designated cost           0                    hold timer                 0.00
 mc router                 1                    mc fast leave              0
 flags

swp2 (2)
 port id                8002                    state                forwarding
 designated root        8000.7072cf8c272c       path cost                  2
 designated bridge      8000.7072cf8c272c       message age timer          0.00
 designated port        8002                    forward delay timer        0.00
 designated cost           0                    hold timer                 0.00
 mc router                 1                    mc fast leave              0
 flags

swp3 (3)
 port id                8003                    state                forwarding
 designated root        8000.7072cf8c272c       path cost                  2
 designated bridge      8000.7072cf8c272c       message age timer          0.00
 designated port        8003                    forward delay timer        8.98
 designated cost           0                    hold timer                 0.00
 mc router                 1                    mc fast leave              0
 flags

To get the groups and bridge port state, use the bridge mdb show command. To display router ports and group information use the bridge -d -s mdb show command:

cumulus@switch:~$ sudo bridge -d -s mdb show
 dev bridge port swp2 grp 234.10.10.10 temp 241.67
 dev bridge port swp1 grp 238.39.20.86 permanent 0.00
 dev bridge port swp1 grp 234.1.1.1 temp 235.43
 dev bridge port swp2 grp ff1a::9 permanent 0.00
 router ports on bridge: swp3

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