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DGS-3610 Series Configuration Guide Chapter 15 IGMP Snooping Configuration
group connected. Currently, there are three versions of IGMP: IGMPv1 is described in rfc
1112, IGMPv2 is described in rfc 2236, and IGMPv3 is described in RFC 3376.
We describe respectively, as below, how the host joins or leaves a multicast in IGMPv1,
IGMPv2 (suppose joining in 224.1.1.1).
In IGMPv1, the host sends the IGMP report packet of 224.1.1.1 to a certain interface on the
router to ask for joining this group. After receiving this request, the interface on the router
forwards the message of the corresponding multicast group for the reason of trusting the
multicast members being existed on the interface. The router interface periodically sends the
IGMP Query message of 224.0.0.1 (all hosts). If the host continues to receive the message
of this group, it shall respond the corresponding IGMP Report packet. If a certain interface
cannot receive the IGMP Report packet of any host, it is believed that there are no multicast
members on this interface, so the message of the corresponding group is not forwarded to
the interface.
IGMPv2 is downward compatible with v1. It extends the message —— adding the IGMP
Leave message, so that the host can initiatively request for leaving the multicast group. In
IGMPv2, the process for the host to join the group is consistent with its process in IGMPv1.
The host sends an IGMP Report packet to request for joining a certain group. The router
periodically sends the IGMP Query message of 224.0.0.1. If the host wants to continue to
receive the message of this group, it should return the response IGMP Report packet. If the
router cannot receive the IGMP Report packet of any host, it will remove this group. In
IGMPv2, the host can also actively leave a certain group. When the host no longer needs a
certain multicast flow, it actively sends the IGMP Leave message to the router and actively
logs out from this group. After receiving the IGMP Leave message, the router sends the
IGMP Query message of the group to determine whether any other hosts in the group need
to receive the multicast information. At this time, if other hosts need to receive the multicast
group, it responds with the IGMP Report packet. If the router fails to receive the response
from any host, it cancels the group.
On the basis of the IGMPV1/V2, the IGMPV3 provides an additional source filtering multicast
function. IGMPv3 to interact with the router is the same as that of IGMPv2. In the IGMP
V1/V2, the host determines to join a group and receive the multicast traffic to the group
address from any source only based on the group address. On the other hand, the host
running the IGMP V3 notifies this host the desired multicast group to join, and also the
addresses of the multicast sources to receive. The host can indicate that it wants to receive
multicast traffic from which sources through a list or an exclusion list. At the same time,
another benefit of the IGMP v3 is that it saves bandwidth to avoid unnecessary, invalid
multicast data traffics from occupying network bandwidth. It is particularly useful in the case
where multiple multicast sources share one multicast address.
Compared with IGMPv2, IGMPv3 specifies two types of packets: Membership Query and
Version 3 Membership Report. There are three types of Membership Query:
General Query: Used to query all the multicast members under the interface:
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