Internet Group Management Protocol (Igmp); Igmp Versions 1 And 2 - D-Link DES-3326 User Manual

24-port NWay Ethernet Switch User's Guide
Internet Group Management Protocol
(IGMP)
Destinations that want to receive multicast packets need to
inform the immediately-neighboring routers. Each node that has
requested to receive multicast packets must become a member of
the multicast group to which these packets are being sent. The
protocol through which hosts communicate this information with
their local routers is called the Internet Group Management
Protocol (IGMP). IGMP is also used by the routers to
periodically check whether the known group members are still
active. If there is more than one multicast router on a given
subnetwork (LAN), one of the routers is elected as the "querier"
and assumes the responsibility of keeping track of the
membership state of the multicast groups which have active
members on its subnetwork. Based on the information obtained
from IGMP, the router can decide whether to forward multicast
packets it receives to its subnetworks or not. After receiving a
multicast packet sent to a certain multicast group, the router
will check and see if there is at least one member of that
particular group on its subnetwork. If that is the case the router
will forward the packet to that subnetwork. Otherwise, it will
discard the multicast packet. Obviously this will be the last
phase of delivering a multicast packet.

IGMP Versions 1 and 2

Fundamental to multicasting is the concept of joining and leaving
multicast groups. The IGMP provides a method through which a
host can join or leave a multicast group. IGMP version 1 is
defined in RFC 1112. IGMP that is considered a part of the IP
layer has a fixed sized packet with no optional data. The format
of an IGMP packet is shown below.
106 Switch Management and Operating Concepts