This documentation is for the extended support release (ESR) version of Cumulus Linux. We will continue to keep this content up to date until 21 February, 2023, when ESR support ends. For more information about ESR, please read this knowledge base article.

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IGMP and MLD Snooping

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

In Cumulus Linux 3.7.4 and later, IGMP and MLD snooping is supported over VXLAN bridges; however, this feature is not enabled by default. To enable IGMP and MLD over VXLAN, see Configure IGMP/MLD Snooping over VXLAN.

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.

/images/download/attachments/8362696/igmp_snoop_diagram.png

Configure IGMP/MLD Snooping over VXLAN

Cumulus Linux 3.7.4 and later supports IGMP/MLD snooping over VXLAN bridges, where VXLAN ports are set as router ports, on Broadcom switches.

Cumulus Linux 3.7.9 and later also supports IGMP/MLD snooping over VXLAN bridges on Spectrum switches. However, in addition to enabling IGMP/MLD snooping over VXLAN, you need to perform an additional configuration step, described below.

To enable IGMP/MLD snooping over VXLAN, run the net add bridge <bridge> mcsnoop yes command:

cumulus@switch:~$ net add bridge mybridge mcsnoop yes
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

Cumulus Networks recommends that you also configure IGMP/MLD querier. See Configure IGMP/MLD Querier below.

To disable IGMP/MLD snooping over VXLAN, run the net add bridge <bridge> mcsnoop no command.

Additional Configuration for Spectrum Switches

For Spectrum switches, the IGMP reports received over VXLAN from remote hosts are not forwarded to the kernel, which, in certain cases, might result in local receivers not responding to the IGMP query. To workaround this issue, you need to apply certain ACL rules to avoid the IGMP report packets being sent across to the hosts:

Add the following lines to the /etc/cumulus/acl/policy.d/23_acl_test.rules file (where <swp> is the port connected to the access host), then run the cl-acltool -i command:

[ebtables]
-A FORWARD -p IPv4 -o #<swp> --ip-proto igmp -j ACCEPT --ip-destination 224.0.0.0/24
-A FORWARD -p IPv4 -o #<swp> --ip-proto igmp -j DROP

DIP-based Multicast Forwarding

DIP-based multicast forwarding is supported on Broadcom switches only.

Cumulus Linux 3.7.10 and earlier performs layer 2 multicast bridging using the destination MAC address (DMAC) of the packet, which is programmed in the layer 2 table of the ASIC. Cumulus Linux 3.7.11 and later provides the option of using IP-based layer 2 multicast forwarding (DIP), where layer 2 multicast packets are forwarded based on the layer 3 forwarding table, using the VLAN as the key.

DIP-based multicast forwarding is a good solution if you want to have a separate bridge domain and multicast flood domain for two groups that map to the same MAC address. In multicast, there can be multiple group addresses that map to the same MAC address as the address is derived from the three octets of the group; out of the allowed multicast range, you have 16 group addresses with the same MAC address.

DIP-based multicast forwarding is also a good solution if you use a group that falls in to the link local address range (for example, 228.0.0.1), which is not forwarded with DMAC-based multicast forwarding.

DIP-based multicast forwarding is not supported with IGMP Snooping over VXLAN or with IPv6 addresses (DMAC-based forwarding is used for IPv6 addresses).

To enable DIP-based multicast forwarding:

  1. Edit the /etc/cumulus/switchd.conf file to set the bridge.dip_based_l2multicast field to TRUE, then uncomment the line.
  2. Restart the switchd service:
cumulus@switch:~$ sudo systemctl restart switchd.service

The following example shows that the bridge.dip_based_l2multicast field is set to TRUE and the line is uncommented in the /etc/cumulus/switchd.conf file:

cumulus@switch:~$ sudo nano /etc/cumulus/switchd.conf
...
# configure IP based forwarding for L2 Multicast
bridge.dip_based_l2multicast = TRUE
...

Configure 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.

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

Troubleshooting

To show 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 show the groups and bridge port state, run the NCLU net show bridge mdb command or the Linux bridge mdb show command. To show detailed router ports and group information, run 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