Internet Group Management Protocol | 413Figure 19-10. Enabling IGMP SnoopingDisabling Multicast FloodingIf the switch receives a multicast packet that has an IP address of a group it has not learned (unregisteredframe), the switch floods that packet out of all ports on the VLAN.On the E-Series, you can configure the switch to only forward unregistered packets to ports on a VLANthat are connected to multicast routers (mrouter ports) using the command no ip igmp snooping flood fromCONFIGURATION mode. When flooding is disabled, if there are no such ports in the VLAN connected toa multicast router, the switch drops the packets.On the C-Series and S-Series, when you configure no ip igmp snooping flood, the system drops the packetsimmediately. The system does not forward the frames on mrouter ports, even if they are present. On theC-Series and S-Series, Layer 3 multicast must be disabled (no ip multicast-routing) in order to disablemulticast flooding.Specifying a Port as Connected to a Multicast RouterYou can statically specify a port in a VLAN as connected to a multicast router using the command ip igmpsnooping mrouter from INTERFACE VLAN mode.View the ports that are connected to multicast routers using the command show ip igmp snooping mrouterfrom EXEC Privilege mode.Configuring the Switch as QuerierHosts that do not support unsolicited reporting wait for a general query before sending a membershipreport. When the multicast source and receivers are in the same VLAN, multicast traffic is not routed, andso there is no querier. You must configure the switch to be the querier for a VLAN so that hosts sendmembership reports, and the switch can generate a forwarding table by snooping.Configure the switch to be the querier for a VLAN by first assigning an IP address to the VLAN interface,and then using the command ip igmp snooping querier from INTERFACE VLAN mode.• IGMP snooping Querier does not start if there is a statically configured multicast router interface in theVLAN.• The switch may lose the querier election if it does not have the lowest IP address of all potentialqueriers on the subnet.FTOS(conf-if-vl-100)#show config!interface Vlan 100no ip addressip igmp snooping fast-leaveshutdownFTOS(conf-if-vl-100)#
