362 | Internet Group Management Protocol (IGMP)w w w . d e l l . c o m | s u p p o r t . d e l l . c o m IGMP version 2IGMP version 2 improves upon version 1 by specifying IGMP Leave messages, which allows hosts tonotify routers that they no longer care about traffic for a particular group. Leave messages reduce theamount of time that the router takes to stop forwarding traffic for a group to a subnet (leave latency) afterthe last host leaves the group. In version 1 hosts quietly leave groups, and the router waits for a queryresponse timer several times the value of the query interval to expire before it stops forwarding traffic.To receive multicast traffic from a particular source, a host must join the multicast group to which thesource is sending traffic. A host that is a member of a group is called a receiver. A host may join manygroups, and may join or leave any group at any time. A host joins and leaves a multicast group by sendingan IGMP message to its IGMP Querier. The querier is the router that surveys a subnet for multicastreceivers, and processes survey responses to populate the multicast routing table.IGMP messages are encapsulated in IP packets, as shown in Figure 18-1.Figure 18-1. IGMP version 2 Packet FormatJoining a Multicast GroupThere are two ways that a host may join a multicast group: it may respond to a general query from itsquerier, or it may send an unsolicited report to its querier.Responding to an IGMP Query1. One router on a subnet is elected as the querier. The querier periodically multicasts (toall-multicast-systems address 224.0.0.1) a general query to all hosts on the subnet.2. A host that wants to join a multicast group responds with an IGMP Membership Report that containsthe multicast address of the group it wants to join (the packet is addressed to the same group). Ifmultiple hosts want to join the same multicast group, only the report from the first host to respondreaches the querier, and the remaining hosts suppress their responses (see Adjusting Query andResponse Timers on page 368 for how the delay timer mechanism works).3. The querier receives the report for a group and adds the group to the list of multicast groups associatedwith its outgoing port to the subnet. Multicast traffic for the group is then forwarded to that subnet.Version(4)IHL TOS(0xc0)Total Length Flags Frag Offset IGMP PacketHeaderChecksumSrc IP AddrTTL(1)Protocol(2)Dest IP Addr Options(Router Alert)PaddingPreamble Start FrameDelimiterDestination MAC Source MAC Ethernet Type IP Packet PaddingfnC0069mpType Max. ResponseTimeGroup AddressChecksumCode: 0x11: Membership Query0x12: IGMP version 1 Membership Report0x16: IGMP version 2 Membership Report0x17: IGMP Leave Group8 bits 16 bitsMay be zero and ignored by hosts forgeneral queries or contain a groupaddress for group-specific queriesFCS
