Configuring L2 Multicast Features 707What 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 thepermanently assigned (well-known) multicast address FF0x::/12 to all ports inthe VLAN, except for MLD packets, which are handled according the MLDsnooping rules.NOTE: It is strongly recommended that users enable IGMP snooping if MLDsnooping is enabled and vice-versa. This is because both IGMP snooping andMLD snooping utilize the same forwarding table, and not enabling both maycause unwanted pruning of protocol packets utilized by other protocols, e.g.OSPFv2.
