15.1.4 Understanding Operation Modes of
IGMP Snooping
DISABLE mode: In this mode, IGMP Snooping is not effective, that is, the switch does not
―snoop‖ the IGMP message between the host and the router or multicast frame when the
broadcast is forwarded within the VLAN.
IVGL operation mode: In this mode, the multicast flows among various VLANs are
independent. The host can only request multicast with the router interface which is located in
the same VLAN with it.
SVGL operation mode: In this mode, the hosts of various VLANs share the same multicast
flow. The host can apply for multicast flow across VLANs. Designate one Multicast VLAN,
and the multicast data flows received in this VLAN can be forwarded to other cross-VLAN
hosts, as shown below. See the figure below.
Figure 15-5
So long as the VID of the multicast data flow is Multicast VLAN (or UNTAG data flow, the
native VLAN of the receiving port is Multicast VLAN), all will be forwarded to the member
port of this multicast address, whether this member port is within this VLAN or not. The VID
of the generated multicast forwarding table will be Multicast VLAN. In the SVGL mode,
except the router interface, for other ports, only when they are in the Multicast VLAN, can the
multicast sent by them be forwarded within the VLAN.
IVGL and SVGL modes can coexist. You can allocate a batch of multicast addresses to
SVGL. Within this batch of multicast addresses, the multicast forwarding tables (GDA table)
are all forwarded across VLANs, while other multicast addresses are forwarded in IVGL
mode.
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