Web Management Guide-R07

Chapter 14 | Multicast Filtering
Layer 2 IGMP (Snooping and Query for IPv4)
– 418 –
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).
Note: A single table maintains a maximum of 511 entries learned for both IGMP
and MLD snooping. When only IGMP snooping is using the table, the maximum
number of entries is 511. When only MLD snooping is using the table, the
maximum number of entries is 255. When both IGMP and MLD are using the table,
the number of possible entries will be limited to less than the maximum.
Note: Only ASM (any-source multicast) mode is supported on IGMPv3 and MLDv2.
SSM (source-specific multicast) on IGMPv3 and MLDv2 is not supported
. If a client
sends an SSM report to the switch, the client will not join the multicast group
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 420) 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