PLANET SGSD-1022 - Manual (Page 207)

User’s Manual of SGSD-1022 / SGSD-1022P
SGSW-2840 / SGSW-2840P
4.9 Multicast
Multicasting is used to support real-time applications such as video conferencing or streaming audio. A multicast server does
not have to establish a separate connection with each client. It merely broadcasts its service to the network, and any hosts that
want to receive the multicast register with their local multicast switch/router. Although this approach reduces the network
overhead required by a multicast server, the broadcast traffic must be carefully pruned at every multicast switch/router it passes
through to ensure that traffic is only passed on the hosts that subscribed to this service.
This Managed Switch uses IGMP (Internet Group Management Protocol) to query for any attached hosts that want to receive
a specific multicast service. It identifies the ports containing hosts requesting to join the service and sends data out to those
ports only. It then propagates the service request up to any neighboring multicast switch/router to ensure that it will continue to
receive the multicast service. This procedure is called multicast filtering.
The purpose of IP multicast filtering is to optimize a switched network's performance, so multicast packets will only be forwarded
to those ports containing multicast group hosts or multicast routers/switches, instead of flooding traffic to all ports in the subnet
(VLAN).
You can also configure a single network-wide multicast VLAN shared by hosts residing in other standard or private VLAN groups,
preserving security and data isolation “Multicast VLAN Registration”.
4.9.1 Layer 2 IGMP (Snooping and Query)
IGMP Snooping and Query – If multicast routing is not supported on other switches in your network, you can use IGMP
Snooping and Query to monitor IGMP service requests passing between multicast clients and servers, and dynamically
configure the switch ports which need to forward multicast traffic.
The Internet Group Management Protocol (IGMP) lets host and routers share information about multicast groups
memberships. IGMP snooping is a switch feature that monitors the exchange of IGMP messages and copies them to the CPU
for feature processing. The overall purpose of IGMP Snooping is to limit the forwarding of multicast frames to only ports that are
a member of the multicast group.
About the Internet Group Management Protocol (IGMP) Snooping
Computers and network devices that want to receive multicast transmissions need to inform nearby routers that they will
become members of a multicast group. The Internet Group Management Protocol (IGMP) is used to communicate this
information. IGMP is also used to periodically check the multicast group for members that are no longer active. In the case
where there is more than one multicast router on a sub network, one router is elected as the ‘queried’. This router then keeps
track of the membership of the multicast groups that have active members. The information received from IGMP is then used to
determine if multicast packets should be forwarded to a given sub network or not. The router can check, using IGMP, to see if
there is at least one member of a multicast group on a given subnet work. If there are no members on a sub network, packets
will not be forwarded to that sub network.
207