Typical Multicast Setup
While Multicast routers route Multicast packets between IP subnets, Multicast- capable Layer 2 switches forward Multicast packets to registered nodes within a LAN or VLAN.
A typical setup involves a router that forwards the Multicast streams between private and/or public IP networks, a switch with Internet Group Membership Protocol (IGMP) snooping capabilities, or Multicast Listener Discovery (MLD) snooping, and a Multicast client that wants to receive a Multicast stream. In this setup, the router sends IGMP queries periodically.
NOTE MLD for IPv6 is derived from the IGMP v2 for IPv4. Even though the description in this section is mostly for IGMP, it also describes coverage of MLD where implied.
These queries reach the switch, which in turn floods the queries to the VLAN, and also learns the port where there is a Multicast router (Mrouter). When a host receives the IGMP query message, it responds with an IGMP Join message saying that the host wants to receive a specific Multicast stream and optionally from a specific source. The switch with the IGMP snooping analyzes the Join messages, and learns that the Multicast stream the host has requested must be forwarded to this specific port. It then forwards the IGMP Join to the Mrouter only. Similarly, when the Mrouter receives an IGMP Join message, it learns the interface from which it received the Join messages that wants to receive a specific Multicast stream. The Mrouter forwards the requested Multicast stream to the interface.
Multicast Operation
In a Layer 2 Multicast service, a Layer 2 switch receives a single frame addressed to a specific Multicast address. It creates copies of the frame to be transmitted on each relevant port.
When the switch is IGMP/MLD-snooping-enabled and receives a frame for a Multicast stream, it forwards the Multicast frame to all the ports that have registered to receive the Multicast stream using IGMP Join messages.
The switch can forward Multicast streams based on one of the following options:
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 or Multicast Listener Discovery (MLD) protocols snooping.
Multicast Registration
Multicast registration is the process of listening and responding to Multicast registration protocols. The available protocols are IGMP for IPv4 and MLD for IPv6.
When IGMP/MLD snooping is enabled in a switch on a VLAN, it analyzes the IGMP/ MLD packets it receives from the VLAN connected to the switch and Multicast routers in the network.
When a switch learns that a host is using IGMP/MLD messages to register to receive a Multicast stream, optionally from a specific source, the switch adds the registration to its Multicast Forwarding Data Base (MFDB).
IGMP/MLD snooping can effectively reduce Multicast traffic from streaming bandwidth-intensive IP applications. A switch using IGMP/MLD snooping only forwards Multicast traffic to the hosts interested in that traffic. This reduction of Multicast traffic reduces the packet processing at the switch, 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.
The following versions are supported: