Dell PowerEdge XL 5133-4 MXL 10/40GbE Switch IO Module FTOS Command Reference - Page 124

www.dell.com | support.dell.com Figure 9-1. IGMP Version 2 Packet Format Preamble Start Frame Destination MAC Delimiter Source MAC Ethernet Type IP Packet Padding FCS Version IHL (4) TOS (0xc0) Total Length Flags Frag Offset TTL (1) Protocol (2) Header Checksum Src IP Addr Dest IP Addr Options (Router Alert) Padding IGMP Packet Type Max. Response Time Checksum 8 bits 16 bits Code: 0x11: Membership Query 0x12: IGMP version 1 Membership Report 0x16: IGMP version 2 Membership Report 0x17: IGMP Leave Group Group Address May be zero and ignored by hosts for general queries or contain a group address for group-specific queries fnC0069mp Joining a Multicast Group There are two ways that a host may join a multicast group: it may respond to a general query from its querier, or it may send an unsolicited report to its querier. • Responding to an IGMP Query • One router on a subnet is elected as the querier. The querier periodically multicasts (to all-multicast-systems address 224.0.0.1) a general query to all hosts on the subnet. • A host that wants to join a multicast group responds with an IGMP membership report that contains the multicast address of the group it wants to join (the packet is addressed to the same group). If multiple hosts want to join the same multicast group, only the report from the first host to respond reaches the querier, and the remaining hosts suppress their responses (for how the delay timer mechanism works, refer to IGMP Snooping). • The querier receives the report for a group and adds the group to the list of multicast groups associated with its outgoing port to the subnet. Multicast traffic for the group is then forwarded to that subnet. • Sending an Unsolicited IGMP Report • A host does not have to wait for a general query to join a group. It may send an unsolicited IGMP membership report, also called an IGMP Join message, to the querier. Leaving a Multicast Group • A host sends a membership report of type 0x17 (IGMP Leave message) to the all routers multicast address 224.0.0.2 when it no longer cares about multicast traffic for a particular group. • The querier sends a group-specific query to determine whether there are any remaining hosts in the group. There must be at least one receiver in a group on a subnet for a router to forward multicast traffic for that group to the subnet. • Any remaining hosts respond to the query according to the delay timer mechanism (refer to IGMP Snooping). If no hosts respond (because there are none remaining in the group), the querier waits a specified period and sends another query. If it still receives no response, the querier removes the group from the list associated with forwarding port and stops forwarding traffic for that group to the subnet. 110 | Internet Group Management Protocol (IGMP)