Unjoined multicast traffic

This feature adds a global IGMP multicast configuration option to the switch that results in each VLAN having a multicast filter. The filter prevents unjoined multicast traffic from being forwarded on interfaces associated with IGMP queriers. Each filter only contains interfaces that are queriers on the same VLAN, so multicast traffic is only flooded on interfaces that contain queriers that are on the same VLAN as the multicast traffic.

On switch bootup, all VLANs that are IGMP-enabled are guaranteed one multicast filter. You can always reboot the switch to recreate this configuration where each IGMP-enabled VLAN has a multicast filter.

NOTE: Joined multicast traffic continues to be forwarded as usual. Reboot the switch after configuring the per-VLAN filter.

Syntax:


igmp filter-unknown-mcast
no igmp filter-unknown-mcast

Enables interface isolation for unjoined multicast groups. IGMP is configured so that each interface with IGMP enabled will have a data-driven multicast filter associated with it, preventing unjoined IP multicast packets from being flooded. A reboot is required for the change to take effect.

Default: Disabled

Enabling the IGMP multicast filter

switch(config)# igmp filter-unknown-mcast
Command will take effect after saving configuration
and reboot.

The following figure shows the multicast traffic being flooded to all queriers on all VLANs; this is the default behavior. The igmp filter-unknown-mcast command has not been executed.

Multicast filter table on distribution switch
VLAN ID Member Ports

0 (all VLANs)

1, 2, 3

Example of unknown multicast traffic flooding on all ports connected to a querier for any VLAN

In the previous figure, igmp filter-unknown-mcast has been configured. The multicast traffic only goes to the querier on the same VLAN as the multicast server.

Multicast filter table on distribution switch
VLAN ID Member Ports

100

1

200

2

300

3

Example of unknown multicast traffic not flooding out ports connected to queriers in separate VLANs

To display the status of IGMP multicast filtering use the show ip igmp command. If the IGMP Filter Unknown Multicast setting is different from the IGMP Filter Unknown Multicast status, a reboot is required to activate the desired setting. This setting will then be reflected in the status.

IGMP unknown multicast filter setting being enabled but not yet activated

switch(config)# show igmp filter-unknown-mcast

IGMP Filter Unknown Multicast: Enabled
IGMP Filter Unknown Multicast Status: Disabled

To display information about IGMP multicast filtering by interface, use the show ip igmp command.