Chapter 24 Multicast
GS2220 Series User’s Guide
224
IP Multicast Addresses
In IPv4, a multicast address allows a device to send packets to a specific group of hosts (multicast
group) in a different subnetwork. A multicast IP address represents a traffic receiving group, not
individual receiving devices. IP addresses in the Class D range (224.0.0.0 to 239.255.255.255) are used for
IP multicasting. Certain IP multicast numbers are reserved by IANA for special purposes (see the IANA
website for more information).
IGMP Snooping
A Switch can passively snoop on IGMP packets transferred between IP multicast routers or 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.
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.
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 through 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 one
query from a router (
X
) or MLD Done or Report message from any upstream port, it will be broadcast to
all connected upstream ports.
Summary of Contents for GS2220-10
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