Considerations when configuring IGMP

In the factory default configuration, IGMP and IGMP snooping are disabled. When enabling IGMP:

  • If multiple VLANs are not configured, configure IGMP on the default VLAN (DEFAULT_VLAN; VID = 1).

  • If multiple VLANs are configured, configure IGMP on a per-VLAN basis for every VLAN where this feature is to be used.

With the factory default setting, multicast data transmitted from the sources will be flooded on all ports in the VLAN. Configuring IGMP snooping avoids flooding and causes the switch to forward data only to the receivers.

The function of the IGMP querier is to poll other IGMP-enabled devices in an IGMP-enabled interface to elicit group membership information. On enabling IGMP, the router performs this function if there is no other device in the interface to act as querier.

Basic steps to configure IGMP:

  1. Configure VLANs.

  2. Configure ports and assign them to the VLANs.

  3. Configure the L3 interface (an interface VLAN/route only port/L3 LAG) and assign an IP address to the interface.

  4. Enable IGMP.

  5. Choose the desired IGMP version. The default is version 3.

IGMP configuration considerations:

  • For IGMP to be operational, the interface has to be administratively up. For interface VLANs, the L2 VLAN has to be up and one of the ports in the VLAN has to be up.

  • The IP address must be assigned for the interface to become querier.

  • A querier is required for proper IGMP operation. For this reason, you must enable IGMP on the L3 Interface. If the querier functionality is not configured or disabled, you must ensure that there is an IGMP querier in the same VLAN.

  • For IGMP snooping to be operational on a VLAN, the VLAN has to be administratively up and at least one port in the VLAN has to be up.

  • If IGMP snooping is enabled on the VLAN, and IGMP is enabled on the interface VLAN, and the configured version does not match, the lowest version is chosen as the operating version.

  • If the switch becomes the querier for a particular interface, then subsequently detects queries transmitted from another device on the same VLAN, the switch ceases to operate as the querier for that interface.

  • The switch automatically ceases querier operation in an IGMP-enabled interface if it detects another querier on the interface. You can also use the switch CLI to disable the querier capability.

  • Multicast traffic will be flooded on the VLAN, if TTL=1 or TTL>255 regardless of IGMP joins and group membership within the VLAN.