Dell Force10 S4820T S4820T Configuration Guide - Page 424

www.dell.com | support.dell.com To receive multicast traffic from a particular source, a host must join the multicast group to which the source is sending traffic. A host that is a member of a group is called a receiver. A host may join many groups, and may join or leave any group at any time. A host joins and leaves a multicast group by sending an IGMP message to its IGMP Querier. The querier is the router that surveys a subnet for multicast receivers, and processes survey responses to populate the multicast routing table. IGMP messages are encapsulated in IP packets, as shown in the following illustration. 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 1. 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. 2. 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 (see Adjusting Query and Response Timers 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 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 1. 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. 424 | Internet Group Management Protocol (IGMP)