D-Link DES-3326 User Manual - (Igmp)
DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts
99
Internet Group Management Protocol
(IGMP)
End users that want to receive multicast packets must be able to
inform nearby routers that they want to become a multicast group
member of the group these packets are being sent to. The
Internet Group Management Protocol (IGMP) is used by multicast
routers to maintain multicast group membership. IGMP is also
used to coordinate between multiple multicast routers that may be
present on a network by electing one of the multicast routers as the
‘querier’. This router then keep track of the membership of
multicast groups that have active members on the network. IGMP
is used to determine whether the router should forward multicast
packets it receives to the subnetworks it is attached to or not. A
multicast router that has received a multicast packet will check to
determine if there is at least one member of a multicast group that
has requested to receive multicast packets from this source. If
there is one member, the packet is forwarded. If there are no
members, the packet is dropped.
IGMP Versions 1 and 2
Users that want to receive multicast packets need to be able to join
and leave multicast groups. This is accomplished using IGMP.
Figure 5-17. IGMP Message Format
The IGMP Type codes are shown below: