Configuring L2 Multicast Features 707When IGMP snooping querier is enabled, the querier switch sends outperiodic IGMP queries that trigger IGMP report messages from the hosts thatwant to receive IP multicast traffic. The IGMP snooping feature listens tothese IGMP reports to identify multicast router ports. If there is anotherquerier in the network and the local querier is in election mode, then thequerier with the lower IP address is elected and the other querier stopsquerying. If the local querier is not in election mode and another querier isdetected, the local querier stops querying.What Is MLD Snooping?In IPv4, Layer 2 switches can use IGMP snooping to limit the flooding ofmulticast traffic by dynamically configuring the multicast forwardingdatabase so that multicast data traffic is forwarded to only those portsassociated with a multicast router or host that has indicated an interest inreceiving a particular multicast group. In IPv6, MLD snooping performs asimilar function.With MLD snooping, IPv6 multicast data is selectively forwarded to a list ofports that want to receive the data instead of being flooded to all ports in aVLAN. This list is constructed in the MFDB by snooping IPv6 multicastcontrol packets. MLD snooping floods multicast data packets until amulticast router port has been identified. MLD snooping forwardsunregistered multicast data packets to IPv6 multicast routers. MLD snoopingdiscovers multicast routers by listening for MLD queries and populates theMFDB.MLD is a protocol used by IPv6 multicast routers to discover the presence ofmulticast listeners (nodes wishing to receive IPv6 multicast packets) on itsdirectly-attached links and to discover which multicast packets are of interestto neighboring nodes. MLD is derived from IGMP; MLD version 1 (MLDv1)is equivalent to IGMPv2, and MLD version 2 (MLDv2) is equivalent toIGMPv3. MLD is a subprotocol of Internet Control Message Protocol version6 (ICMPv6), and MLD messages are a subset of ICMPv6 messages.The switch snoops both MLDv1 and MLDv2 protocol packets and forwardsIPv6 multicast data based on destination IPv6 multicast MAC addresses(33:33::). The switch floods multicast control plane traffic addressed to theNOTE: Without an IP-multicast router on a VLAN, you must configure anotherswitch as the IGMP querier so that it can send queries.
