In a Layer 2 multicast network, you can enable Layer 2 querier on one of the switches in the VLAN. IGMP Layer 2 querier provides the IGMP querier function so that the switch can provide the recurring queries that maintain IGMP groups when you do not use multicast routing for multicast traffic.
In a multicast network, if you only need to use Layer 2 switching for the multicast traffic, you do not need multicast routing. However, you must have an IGMP querier on the network for multicast traffic to flow from sources to receivers. A multicast router provides the IGMP querier function. You can also use the IGMP Layer 2 Querier feature to provide a querier on a Layer 2 network without a multicast router.
The Layer 2 querier function originates queries for multicast receivers, and processes the responses accordingly. On the connected Layer 2 VLANs, IGMP snoop continues to provide services as normal. IGMP snoop responds to queries and identifies receivers for the multicast traffic.
You must enable Layer 2 querier and configure an IP address for the querier before it can originate IGMP query messages. If a multicast router exists on the network, the switch automatically disables the Layer 2 querier.
In a Layer 2 multicast network, enable Layer 2 querier on only one of the switches in the VLAN. A Layer 2 multicast domain supports only one Layer 2 querier. No querier election exists.
IGMP Snooping enables Layer 2 switches in the network to examine IGMP control protocol packets exchanged between downstream hosts and upstream routers.
When Layer 2 switches examine the IGMP control protocol packets, they:
Generate the Layer 2 MAC forwarding tables used for further switching sessions
Regulate the multicast traffic to prevent it from flooding the Layer 2 segment of the network
IGMP Layer 2 Querier uses IGMP to learn which groups have members on each of the attached physical networks, and it maintains a list of multicast group memberships for each attached network and a timer for each membership. In this case, multicast group memberships means the presence of at least one member of a multicast group on a given attached network, not a list of all of the members.
IGMP Layer 2 Querier can assume one of two roles for each of the attached networks:
Querier
Non-Querier
After you enable IGMP Layer 2 Querier, the system assumes it is a multicast router, so it sends the General Query, Group Specific/Group, and Source Specific Query when Leave/BLOCK messages are received. IGMP queries are required to maintain an IGMP group.
Note
Group Specific When Leave does not apply to IGMPv1.