Layer 2 multicast enables Layer 2 devices to recognize multicast group addresses and record the mappings between multicast group addresses and relevant interfaces in forwarding tables. The Layer 2 devices then forward multicast data based on the forwarding tables. Multicast group addresses can be multicast IP addresses or mapped multicast MAC addresses. Upon receiving multicast packets, the Layer 2 devices search the forwarding tables for downstream interfaces based on group addresses of the multicast packets and forward the multicast packets to relevant downstream interfaces.
After receiving multicast packets from the upstream device, a switch at the edge of the access network forwards the multicast packets to multicast receivers. If the switch is not configured with the Layer 2 multicast function, when receiving the multicast packets, it has to broadcast the multicast packets in the broadcast domain to which the multicast packets belong. In this case, all hosts, including group members and non-group members in the broadcast domain, can receive the multicast packets. This wastes network bandwidth and adversely affects network security.
Layer 2 multicast mainly has the following functions:
IGMP snooping: IGMP snooping generates a multicast forwarding table by intercepting IGMP packets exchanged between an upstream device to hosts, thus managing and controlling the forwarding of multicast packets. In this manner, on-demand multicast packet transmission is implemented at the data link layer.
Multicast VLAN replication: After multicast VLAN replication is configured, the upstream device sends multicast data only to a multicast VLAN and the multicast VLAN then replicates the multicast data on a switch, thereby saving network bandwidth.
Static Layer 2 multicast: A Layer 2 multicast forwarding table is manually configured in which interfaces and multicast address entries are statically bound. In this manner, multicast packets can be forwarded to hosts that need to receive multicast data.
Layer 2 multicast instance: A Layer 2 multicast instance helps transmit only one copy of the required multicast data flow on a VLAN or VPLS network and replicate the data flow to users in different domains. Deploying Layer 2 multicast instances greatly saves network bandwidth.
Rapid multicast data forwarding on a backup device: On a VPLS network, a backup device can be enabled to forward multicast data flows so that multicast data flows can be rapidly switched to the backup device if the master link or master device fails.
