Multicast Listener Discovery and Source-specific Multicast Protocols

Multicast Listener Discovery Version 2 (MLDv2) protocol is supported. IPv6 routers use the MLDv2 protocol to discover multicast listeners, or nodes that wish to receive multicast packets on directly attached links. MLDv2 supports source filtering, the ability of a node to send reports on traffic that is from a specific source address or from all multicast addresses except the specified source addresses. The information is then provided to the source-specific multicast (SSM) routing protocols such as PIM-SSM.

The IPv6 router stores a list of multicast addresses for each attached link. For each multicast address, the IPv6 router stores a filter mode and a source list. The filter mode is set to INCLUDE if all nodes in the source list for a multicast address are in the INCLUDE state. If the filter mode is INCLUDE, then only traffic from the addresses in the source list is allowed. The filter mode is set to EXCLUDE if at least one of the nodes in the source list is in an EXCLUDE state. If the filter mode is EXCLUDE, traffic from nodes in the source list is denied and traffic from other sources is allowed.

The source list and filter mode are created when the IPv6 querier router sends a query. The querier router is the one with the lowest source IPv6 address. It sends out any of the following queries:

  • General query - The querier sends this query to learn all multicast addresses that need to be listened to on an interface.
  • Address specific query - The querier sends this query to determine if a specific multicast address has any listeners.
  • Address specific and source specific query - The querier sends this query to determine if specified sources of a specific multicast address have any listeners.

In response to these queries, multicast listeners send the following reports:

  • Current state - This report specifies the source list for a multicast address and whether the filter mode for that source list is INCLUDE or EXCLUDE.
  • Filter-mode change - This report specifies if there has been a change to the filter mode for the source list and provides a new source list.
  • Source list change - This report specifies the changes to the source list.