C
HAPTER
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| Multicast Filtering
Layer 2 IGMP (Snooping and Query)
– 389 –
across different subnetworks. Therefore, when PIM routing is enabled for a
subnet on the switch, IGMP is automatically enabled.
Figure 226: IGMP Protocol
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 IGMP Query
(page 391) 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.
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.
Network core
(multicast routing)
Edge switches
(snooping and query)
Switch to end nodes
(snooping on IGMP clients)
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