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DES-7200 Configuration Guide Chapter 8 IGMP Snooping
Configuration
8-6
8.1.7 Understanding Relationship between IGMP Snooping and QinQ
After IGMP Snooping is enabled and dot1q-tunnel port is configured on the device,
IGMP packets received from dot1q-tunnel port will be handled in two ways
through IGMP Snooping:
1st way: Create multicast entries on the VLAN to which IMGP packets belong,
and forward IMGP packets on such VLAN. For example: It is assumed that IGMP
Snooping has been enabled on the device; port A is a dot1q-tunnel port; the
default VLAN of port A is VLAN 1, and packets from VLAN 1 and VLAN 10 can
pass through port A. When multicast requests of VLAN 10 are sent to port A,
IGMP Snooping will create the multicast entry of VLAN 10 and forward the
multicast requests to the router port of VLAN 10.
2nd way: Create multicast entries on the default VLAN to which dot1q-tunnel
belong, and forward multicast packets on the default VLAN of dot1q-tunnel port
after inserting the VLAN Tag of the default VLAN of dot1q-tunnel port. For
example: It is assumed that IGMP Snooping has been enabled on the device; port
A is a dot1q-tunnel port; the default VLAN of port A is VLAN 1, and packets from
VLAN 1 and VLAN 10 can pass through port A. When multicast requests of VLAN
10 are sent to port A, IGMP Snooping will create the multicast entry of VLAN 1
and insert the VLAN Tag of VLAN 1 into multicast requests before forwarding the
multicast requests to the router port of VLAN 1.
By default, the 2nd way is used.
8.1.8 Understanding IGMP Snooping Querier
In a multicast network running IGMP, a Layer-3 multicast device acting as the
IGMP querier is responsible for sending IGMP general queries, so that all Layer-3
multicast devices can establish and maintain multicast forwarding entries, thus to
forward multicast traffic correctly at the network layer.
However, in a network without layer-3 multicast device, a layer-2 multicast device
does not support IGMP, and therefore cannot realize the relevant functions of
IGMP querier. By enabling IGMP snooping on a layer-2 device, the layer-2 device
can establish and maintain multicast forwarding entries at the data link layer, thus
to forward multicast traffic correctly at the data link layer.
8.1.9 Understanding Multicast VLAN
As shown in Figure 3, in the traditional multicast programs-on-demand mode,
when hosts, Host A, Host B and Host C, belonging to different VLANs require
multicast programs-on-demand service, the multicast router needs to copy the
multicast traffic in each VLAN as multicast snooping is only carried out in the
VLAN. This results in not only waste of network bandwidth but also extra burden
on the Layer 3 device.
To solve this problem, we can configure multicast VLAN feature on the switch
(namely IMGP Snooping will be running in SVGL mode or hybrid mode), which
means that the VLANs to which these hosts belong will be configured as the
sub-VLANs of a multicast VLAN. In this way, the multicast router needs to
replicate the multicast traffic only in the multicast VLAN instead of making a
separate copy of the multicast traffic in each user VLAN. This lessens the burden
of the Layer 3 device.
When running on the multicast VLAN, the master multicast VLAN (namely SVGL
VLAN) and the multicast address of multicast VLAN must be specified for the
devices. Meanwhile, the sub-VLANs associated with the multicast VLAN may
also need to be specified. Only the traffic from master multicast VLAN can be