Configuring L2 Multicast Features 705This approach works well for broadcast packets that are intended to be seen orprocessed by all connected nodes. In the case of multicast packets, however,this approach could lead to less efficient use of network bandwidth,particularly when the packet is intended for only a small number of nodes.Packets will be flooded into network segments where no node has any interestin receiving the packet.What Is IGMP Snooping?IGMP snooping allows the switch to snoop on IGMP exchanges betweenhosts and multicast routers. The IGMP snooping feature complies with RFC4541. When a switch “sees” an IGMP report from a host for a given multicastaddress, the switch adds the host's interface/VLAN to the L2 multicast groupforwarding table and floods the report to all ports in the VLAN. When theswitch sees a leave message for the group, it removes the host interface/VLANfrom the L2 multicast group forwarding table.IGMP snooping learns about multicast routers by listening for the followingmessages:• IGMP Membership queries• PIMv1 hellos• PIMv2 hellos• DVMRP probesGroup addresses that fall into the range 224.0.0.x are never pruned by IGMPsnooping—they are always flooded to all ports in the VLAN. Note that thisflooding is based on the IP address, not the corresponding 01-00-5e-00-00-xxMAC address.When a multicast router is discovered, its interface is added to the interfacedistribution list for all multicast groups in the VLAN. If a switch is connectedto a multicast source and no client, the switch filters the traffic from thatgroup to all interfaces in the VLAN. If the switch sees an IGMP join from ahost in the same VLAN, then it forwards the traffic to the host. Likewise, ifthe switch sees a multicast router in the VLAN, it forwards the group to themulticast router and does not flood in the VLAN. There is a user option tocause the switch to flood multicast sources in the VLAN if no multicastclients are present. This option should be enabled when deploying the switch
