KB.15.15

For example, if several employees engage in a desktop conference across the network, they all

need application software on their computers. At the start of the conference, the software on all

the computers determines a multicast address of, for example, FF3E:30:2001:DB8::101 for the

conference. Then any traffic sent to that address can be received by all computers listening on that

address.

General operation

Multicast communication can take place without MLD, and by default, MLD is disabled. In that

case, if a switch receives a packet with a multicast destination address, it floods the packet to all

ports in the same VLAN (except the port that it came in on), as shown in Figure 1 (page 80). Any

network nodes that are listening to that multicast address will see the packet; all other hosts ignore

the packet.

Figure 1 Without MLD, multicast traffic is flooded to all ports

When MLD snooping is enabled on a VLAN, the switch acts to minimize unnecessary multicast

traffic. If the switch receives multicast traffic destined for a given multicast address, it forwards that

traffic only to ports on the VLAN that have MLD hosts for that address, as shown in Figure 2 (page

81). It drops that traffic for ports on the VLAN that have no MLD hosts (except for a few special

cases explained below).

80 Multicast Listener Discovery (MLD) Snooping