Dell Converged Enhanced Ethernet Administrator’s Guide 12753-1002116-01Chapter15Configuring IGMPIn this chapter• About IGMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127• Configuring IGMP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128• Configuring IGMP snooping querier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128• Monitoring IGMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129About IGMPMulticast Control packet and Data Forwarding through a Layer-2 switch configured with VLANs ismost easily achieved by Layer-2 forwarding of received Multicast Packets on all the member portsof the VLAN interfaces. However, this simple approach is not bandwidth efficient, because only asubset of member ports may be connected to devices interested in receiving those Multicastpackets. In the worst case scenario the data would get forwarded to all port members of a VLANwith a large number of member ports (for example, all 24 ports), even if only a single VLAN memberis interested in receiving the data. Such scenarios can lead to loss of throughput for a switch thatgets hit by a high rate of Multicast Data Traffic.Internet Group Management Protocol (IGMP) snooping is a mechanism by which a Layer-2 switchcan effectively address this issue of inefficient Multicast Forwarding to VLAN port members.Snooping involves “learning” forwarding states for Multicast Data traffic on VLAN port membersfrom the IGMP control (Join/Leave) packets received on them. The Layer-2 switch also provides fora way to configure forwarding states statically through the CLI.NOTEDell Fabric OS supports IGMPv1 and IGMPv2.Active IGMP snoopingIGMP snooping is normally passive by nature, as it simply monitors IGMP traffic without filtering.However, active IGMP snooping actively filters IGMP packets to reduce load on the multicast router.Upstream traffic is filtered so that only the minimal quantity of information is sent. The switchensures the router only has a single entry for the VLAN, regardless of the number of active listenersdownstream.In active IGMP snooping, the router only knows about the most recent member of the VLAN. If thereare two active listeners in a VLAN and the original member drops from the VLAN, the switchdetermines that the router does not need this information as the status of the VLAN remainsunchanged. However the next time there is a routine query from the router, the switch will forwardthe reply from the remaining host to prevent the router from assuming there are no active listeners.
