7.8.1.0 - Force10 Networks

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 Figure 195.
Figure 195 IGMP version 2 Packet Format
Version
(4)
IHL TOS
(0xc0)
Preamble Start Frame
Delimiter
Total Length Flags Frag Offset
Joining a Multicast Group
Destination MAC Source MAC Ethernet Type IP Packet Padding
TTL
(1)
Protocol
(2)
Header
Checksum
332 Internet Group Management Protocol
Src IP Addr
Dest IP Addr Options
(Router Alert)
FCS
Padding
Type Max. Response
Time
Code: 0x11: Membership Query
0x12: IGMP version 1 Membership Report
0x16: IGMP version 2 Membership Report
0x17: IGMP Leave Group
Checksum
8 bits 16 bits
IGMP Packet
Group Address
May be zero and ignored by hosts for
general queries or contain a group
address for group-specific queries
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.
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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 on page 338 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.