Linksys LGS308 User Manual page 111

The device can forward Multicast streams based on one of the following options:
• Multicast MAC Group Address
IP Multicast Group Address (G)
•
• A combination of the source IP address (S) and the destination IP Multicast Group Address
(G) of the Multicast packet.
• One of these options can be configured per VLAN.
The system maintains lists of Multicast groups for each VLAN, and this manages the Multicast
information that each port should receive. The Multicast groups and their receiving ports can be
configured statically or learned dynamically using IGMP snooping.
Multicast registration is the process of listening and responding to Multicast registration
protocols. The available protocols are IGMP for IPv4.
When IGMP snooping is enabled in a device on a VLAN, it analyzes the IGMP packets it receives
from the VLAN connected to the device and Multicast routers in the network.
When a device learns that a host is using IGMP messages to register to receive a Multicast
stream, optionally from a specific source, the device adds the registration to its Multicast
Forwarding Data Base (MFDB).
IGMP snooping can effectively reduce Multicast traffic from streaming bandwidth- intensive IP
applications. A device using IGMP snooping only forwards Multicast traffic to the hosts interested
in that traffic. This reduction of Multicast traffic reduces the packet processing at the device, and
also reduces the workload of the end hosts, since they do not have to receive and filter all of the
Multicast traffic generated in the network. This device supports IGMP v1/v2/v3.
Multicast Address Properties
Each IPv4 Multicast address is in the address range 224.0.0.0 to 239.255.255.255.
To map an IP Multicast group address to a Layer 2 Multicast address for IPv4, take the 23 low-
order bits from the IPv4 address, and add them to the 01:00:5e prefix. By standard, the upper
nine bits of the IP address are ignored. IP addresses that differ only in the value of the upper nine
bits are mapped to the same Layer 2 address because the lower 23 bits are identical. For
example, 234.129.2.3 is mapped to a MAC Multicast group address 01:00:5e:01:02:03. Up to
32 IP Multicast group addresses can be mapped to the same Layer 2 address.
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