Multicast Filtering
233
Instruction Manual - NXA-ENET8-POE+
Multicast Filtering
This chapter describes how to configure the following multicast services:
IGMP Snooping
- Configures snooping and query parameters.
Filtering and Throttling
- Filters specified multicast service, or throttles the maximum of multicast groups allowed on an
interface.
MLD Snooping
- Configures snooping and query parameters for IPv6.
Overview
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 to the hosts which subscribed to this service.
This switch can use Internet Group Management Protocol (IGMP) to filter multicast traffic. IGMP Snooping can be used to passively
monitor or snoop on exchanges between attached hosts and an IGMP-enabled device, most commonly a multicast router. In this
way, the switch can discover the ports that want to join a multicast group, and set its filters accordingly.
If there is no multicast router attached to the local subnet, multicast traffic and query messages may not be received by the switch.
In this case (Layer 2) IGMP Query can be used to actively ask the attached hosts if they want to receive a specific multicast service.
IGMP Query thereby 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.
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.
Layer 2 IGMP (Snooping and Query for IPv4)
IGMP Snooping and Query - If multicast routing is not supported on other switches in your network, you can use IGMP Snooping
and IGMP Query (page 234) to monitor IGMP service requests passing between multicast clients and servers, and dynamically
configure the switch ports which need to forward multicast traffic. IGMP Snooping conserves bandwidth on network segments
where no node has expressed interest in receiving a specific multicast service. For switches that do not support multicast routing,
or where multicast routing is already enabled on other switches in the local network segment, IGMP Snooping is the only service
required to support multicast filtering.
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. For IGMPv1/v2 hosts, the source address of a channel is always null
(indicating that any source is acceptable), but for IGMPv3 hosts, it may include a specific address when requested.
FIG. 275
Multicast Filtering Concept
Unicast
Flow
Multicast
Flow