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DES-7200 Configuration Guide Chapter 3 IGMP
Configuration
3-1
3 IGMP Configuration
3.1 IGMP Overview
IPv4 multicast refers to a network technology that forwards packets to more than
one receiver through a multicast flow. Only the hosts joining the group can receive
the packets from the specific multicast group. Multicast can save network
bandwidth greatly for there is only single packet transmitting on any link of the
network, no matter how many receivers are deployed.
Multicast uses Class-D network address specified by IANA. The highest bits of
Class-D network address are 1110. So, the Class-D network address is in the
range of 224.0.0.0 to 239.255.255.255. However, not all addresses in this range
can be used by users. The addresses in the range 224.0.0.1 to 224.0.0.255 are
reserved for protocols. For instance, 224.0.0.1 indicates all multicast host
addresses and 224.0.0.2 indicates all multicast device addresses.
Any hosts, no matter whether they are multicast group member or not, can be the
multicast source. However, only the multicast group member can receive the
multicast frame. The multicast group member is able to dynamically join in or
leave the group. The forwarding of multicast frame in the network is processed by
the multicst device with multicast routing protocol enabled.
To enable IPv4 multicast, multicast hosts and devices must support IGMP. This
protocol is used by the host to notify the multicast device of the multicast
membership of the network they connect to determine how to forward multicast
traffic. By using the information obtained from the IGMP, the device can maintain
an interface and group-based multicast member list. The multicast member list is
activated only when at least one host of an interface is a member of the group.
IGMPv1, IGMPv2 and IGMPv3 are supported at present. On the basis of IGMPv1,
IGMPv2 has the leaving message so that the host can actively request to leave a
multicast group. IGMP activities fall into two parts: host activity and device activity.
3.1.1 IGMPV1
There are only two types of messages defined in IGMP Version 1:
Membership query
Membership report
A host sends a report packet to join a group, and the router sends the query
packet at periodical intervals to ensure that a group has at least one host. When a
group contains no host, the router will delete that group.
3.1.2 IGMPV2
In Version 2, there are only four types of packets:
Membership query