User Manual

Figure 10. IGMP Version 2 Packet Format

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.

IGMP Version 3

Conceptually, IGMP version 3 behaves the same as version 2. However, there are differences:

• Version 3 adds the ability to filter by multicast source, which helps the multicast routing protocols

avoid forwarding traffic to subnets where there are no interested receivers.

IGMP Overview