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User’s Manual of SGSD-1022 / SGSD-1022P
SGSW-2840 / SGSW-2840P
205
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.
When using IGMPv3 snooping, service requests from IGMP Version 1, 2 or 3 hosts are all forwarded to the upstream router as
IGMPv3 reports. The primary enhancement provided by IGMPv3 snooping is in keeping track of information about the specific
multicast sources which downstream IGMPv3 hosts have requested or refused. The switch maintains information about both
multicast groups and channels, where a group indicates a multicast flow for which the hosts have not requested a specific
source (the only option for IGMPv1 and v2 hosts unless statically configured on the switch), and a channel indicates a flow for
which the hosts have requested service from a specific source.
Only IGMPv3 hosts can request service from a specific multicast source. When downstream hosts request service from a
specific source for a multicast service, these sources are all placed in the Include list, and traffic is forwarded to the hosts from
each of these sources. IGMPv3 hosts may also request that service be forwarded from all sources except for those specified. In
this case, traffic is filtered from sources in the Exclude list, and forwarded from all other available sources.