Lantronix SISPM1040-xxxx-L3 Web User Guide
33856 Rev. A https://www.lantronix.com/ 167
MLD Snooping
MLD (Multicast Listener Discovery for IPv6) is used by IPv6 routers to discover multicast listeners on a directly
attached link, much as IGMP is used in IPv4. The protocol is embedded in ICMPv6 instead of using a separate
protocol.
A network node that acts as a source of IPv6 multicast traffic is only an indirect participant in MLD snooping;
it just provides multicast traffic, and MLD doesn’t interact with it. Note, however, that in an application like desktop
conferencing a network node may act as both a source and an MLD host; but MLD interacts with that node only in
its role as an MLD host.
A source node creates multicast traffic by sending packets to a multicast address. In IPv6, addresses with the first
eight bits set (that is, “FF” as the first two characters of the address) are multicast addresses, and any node that
listens to such an address will receive the traffic sent to that address. Application software running on the source
and destination systems cooperates to determine what multicast address to use. Note that this is a function of the
application software, not of MLD.
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.
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