Multicast Listener Discovery (MLD) Snooping

Introduction to MLD Snooping

Introduction to MLD Snooping

There are several roles that network devices may play in an IPv6 multicast environment:

■MLD host—a network node that uses MLD to “join” (subscribe to) one or more multicast groups

■multicast router—a router that routes multicast traffic between sub- nets

■querier—a switch or multicast router that identifies MLD hosts by sending out MLD queries, to which the MLD hosts respond

Curiously enough, 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. Appli­ cation 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.)

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, say, 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.

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