How Igmp Operates - SMC Networks SMC6624M Management Manual

Configuring Advanced Features
Multimedia Traffic Control with IP Multicast (IGMP)
all IGMP traffic to all ports, which can cause unnecessary bandwidth usage
on ports not belonging to multicast groups. Enabling IGMP allows the ports
to detect IGMP queries and report packets and manage IP multicast traffic
through the switch.
IGMP is useful in multimedia applications such as LAN TV, desktop confer-
encing, and collaborative computing, where there is multipoint communica-
tion; that is, communication from one to many hosts, or communication
originating from many hosts and destined for many other hosts. In such
multipoint applications, IGMP will be configured on the hosts, and multicast
traffic will be generated by one or more servers (inside or outside of the local
network). Switches in the network (that support IGMP) can then be config-
ured to direct the multicast traffic to only the ports where needed. If multiple
VLANs are configured, you can configure IGMP on a per-VLAN basis.
Enabling IGMP allows the ports to detect IGMP queries and report packets
and manage IP multicast traffic through the switch. If no other querier is
detected, the switch will then also function as the querier. (If you need to
disable the querier feature, you can do so through the IGMP configuration
MIB. Refer to "Changing the Querier Configuration Setting" on page 9-99.)
N o t e
IGMP configuration on the SMC6624M operates at the VLAN context level. If
you are not using VLANs, then configure IGMP in VLAN 1 (the default VLAN)
context. Some IGMP features require an IP address and subnet mask for any
VLAN used for IGMP traffic (see "IGMP Operation With or Without IP Address-
ing" on page 9-93). If the switch relies on DHCP or Bootp to acquire an IP
address, ensure that an IP addressing has been assigned to the appropriate
VLANs by using show ip or by viewing the menu interface "Management
Address Information" screen (page 10-5).

How IGMP Operates

The Internet Group Management Protocol (IGMP) is an internal protocol of
the Internet Protocol (IP) suite. IP manages multicast traffic by using
switches, multicast routers, and hosts that support IGMP. A set of hosts,
routers, and/or switches that send or receive multicast data streams to or from
the same source(s) is termed a multicast group, and all devices in the group
use the same multicast group address. The multicast group running version 2
of IGMP uses three fundamental types of messages to communicate:
I
9-88
Query: A message sent from the querier (multicast router or switch)
asking for a response from each host belonging to the multicast group. If
a multicast router supporting IGMP is not present, then the switch must
assume this function in order to elicit group membership information