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Layer 2 IGMP (Snooping and Query for IPv4)
Figure 203: IGMP Protocol
IGMP Snooping and Query – If multicast routing is not supported on other switches
in your network, you can use IGMP Snooping and IGMP Query
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.
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 any source except for those specified. In this case, traffic is filtered
from sources in the Exclude list, and forwarded from all other available sources.
Layer 2 IGMP (Snooping and Query for IPv4)
Network core
(multicast routing)
Edge switches
(snooping and query)
Switch to end nodes
(snooping on IGMP clients)
– 311 –
Chapter 13
| Multicast Filtering
(page
313) to
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