IGMP Layer 2 Querier

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.

Overview

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

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:

IGMP Layer 2 Querier and IGMP interaction

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:

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

Note

Group Specific When Leave does not apply to IGMPv1.