80 Switch FeaturesLayer 2 Multicast FeaturesFor information about configuring L2 multicast features, see "Configuring L2Multicast Features" on page 713.MAC Multicast SupportMulticast service is a limited broadcast service that allows one-to-many andmany-to-many connections. In Layer 2 multicast services, a single frameaddressed to a specific multicast address is received, and copies of the frameto be transmitted on each relevant port are created.IGMP SnoopingInternet Group Management Protocol (IGMP) Snooping is a feature thatallows a switch to forward multicast traffic intelligently on the switch.Multicast IP traffic is traffic that is destined to a host group. Host groups areidentified by class D IP addresses, which range from 224.0.0.0 to239.255.255.255. Based on the IGMP query and report messages, the switchforwards traffic only to the ports that request the multicast traffic. Thisprevents the switch from broadcasting the traffic to all ports and possiblyaffecting network performance.IGMP Snooping QuerierWhen Protocol Independent Multicast (PIM) and IGMP are enabled in anetwork with IP multicast routing, the IP multicast router acts as the IGMPquerier. However, if the IP-multicast traffic in a VLAN needs to be Layer 2switched only, an IP-multicast router is not required. The IGMP SnoopingQuerier can perform the IGMP snooping functions on the VLAN.MLD SnoopingIn IPv4, Layer 2 switches can use IGMP Snooping to limit the flooding ofmulticast traffic by dynamically configuring Layer 2 interfaces so thatmulticast traffic is forwarded to only those interfaces associated with IPmulticast address.In IPv6, MLD snooping performs a similar function. With MLD snooping,IPv6 multicast data is selectively forwarded to a list of ports intended toreceive the data (instead of being flooded to all of the ports in a VLAN). Thislist is constructed by snooping IPv6 multicast control packets.
