140 • DM NVX® 4K60 4:2:0 Network AV Encoders and Decoders
Product Manual — Doc. 9219A
Appendix: IGMP Snooping
A DM NVX device sends IGMP join and leave messages. The network switch port that connects to
a DM NVX device must be enabled with IGMPv2 or IGMPv3 snooping to prevent the switch from
flooding the multicast destination address traffic to all other connected ports. The multicast
destination address that is configured for the DM NVX device must be within the range of
qualified addresses. An upstream device such as a layer 3 router or switch periodically sends the
IGMP General Query messages to hosts in order to maintain group membership state
information. These queries can be either general or group-specific queries. The host responds to
queries with IGMP membership reports. The host running IGMPv2 or IGMPv3 may also send a
Leave Group message to routers or switches in order to withdraw from the group.
NOTES:
l
DM NVX devices do not support random-timer and source-specific queries.
l
As a host, a DM NVX device configured for support of IGMPv3 is compatible with a
network switch (IGMP querier) that is configured for IGMPv2.
IGMP snooping switches build forwarding lists by listening for and, in some cases, intercepting
IGMP messages. Although the software processing the IGMP messages may maintain state
information based on the full IP group addresses, the forwarding tables are typically mapped to
link layer addresses as shown in the following example.
Example of Forwarding Table
Multicast MAC Address
Member Ports
01-00-5E-00-00-01
2, 7
01-00-5E-01-02-03
1, 2, 3, 7
01-00-5E-23-E2-05
1, 4
Because only the least significant 23 bits of the IP address are mapped to Ethernet addresses
(RFC 1112), there is a loss of information when forwarding solely on the destination MAC
address. For example, IP addresses 224.0.0.123 and 239.128.0.123 and similar IP multicast
addresses all map to MAC address 01-00-5e-00-00-7b for Ethernet. As a result, IGMP snooping
switches may collapse IP multicast group memberships into a single Ethernet multicast
membership group.
In addition to building and maintaining lists of multicast group memberships, the snooping
switch must also maintain a list of multicast routers. When multicast packets are forwarded, the
packets should be forwarded not only on ports that have expressed joins using IGMP but also on
ports to which multicast routers are attached.
