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7-4
Multicast Listener Discovery (MLD) Snooping
Introduction to MLD Snooping
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). Any network nodes that are listening
to that muticast address will see the packet; all other hosts ignore the packet.
Figure 7-1. 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. It drops that traffic for ports on the
VLAN that have no MLD hosts (except for a few special cases explained
below).
Figure 7-2. With MLD snooping, traffic is sent to MLD hosts.
Switch
Source
Listener
Listener
MLD disabled
Switch
Source
Listener
(MLD host)
Listener
(MLD host)
MLD snooping enabled