Chapter 22 Multicast
GS3700/XGS3700 Series User’s Guide
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22.1.3 IGMP Snooping
The Switch can passively snoop on IGMP packets transferred between IP multicast routers/switches
and IP multicast hosts to learn the IP multicast group membership. It checks IGMP packets passing
through it, picks out the group registration information, and configures multicasting accordingly.
IGMP snooping allows the Switch to learn multicast groups without you having to manually
configure them.
The Switch forwards multicast traffic destined for multicast groups (that it has learned from IGMP
snooping or that you have manually configured) to ports that are members of that group. IGMP
snooping generates no additional network traffic, allowing you to significantly reduce multicast
traffic passing through your Switch.
22.1.4 IGMP Snooping and VLANs
The Switch can perform IGMP snooping on up to 16 VLANs. You can configure the Switch to
automatically learn multicast group membership of any VLANs. The Switch then performs IGMP
snooping on the first 16 VLANs that send IGMP packets. This is referred to as auto mode.
Alternatively, you can specify the VLANs that IGMP snooping should be performed on. This is
referred to as fixed mode. In fixed mode the Switch does not learn multicast group membership of
any VLANs other than those explicitly added as an IGMP snooping VLAN.
22.1.5 MLD Snooping-Proxy
MLD snooping-proxy is a ZyXEL-proprietary feature. IPv6 MLD proxy allows only one upstream
interface on a switch, while MLD snooping-proxy supports more than one upstream port on a
switch. The upstream port in MLD snooping-proxy can report group changes to a connected
multicast router and forward MLD messages to other upstream ports. This helps especially when
you want to have a network that uses STP to provide backup links between switches and also
performs MLD snooping and proxy functions. MLD snooping-proxy, like MLD proxy, can minimize
MLD control messages and allow better network performance.
In MLD snooping-proxy, if one upstream port is learned via snooping, all other upstream ports on
the same device will be added to the same group. If one upstream port requests to leave a group,
all other upstream ports on the same device will also be removed from the group.
In the following MLD snooping-proxy example, all connected upstream ports (1 ~7) are treated as
one interface. The connection between ports 8 and 9 is blocked by STP to break the loop. If there is
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