This chapter describes how to configure Multicast Listener Discovery (MLD).

The Multicast Listener Discovery (MLD) is used by IP hosts to report their multicast group memberships to any immediately-neighboring multicast routers.

Using the information obtained through MLD, the router maintains a list of multicast group on a per-interface basis. The routers that receive these MLD packets send multicast data that they receive for requested groups out the network segment of the known receivers.

By default, when PIMv6 is enabled on an interface, MLD version 2 is enabled. MLD can be enabled on an interface explicitly.

OcNOS supports MLDv1 and MLDv2. By default, OcNOS enables MLDv2 when PIMv6 is enabled on an interface.

MLDv2 includes the following key changes from MLDv1:

MLD works on the premise of three major packets exchange between MLD enabled routers and hosts, interested in joining a particular group.

Once MLD is enabled or PIMv6 is enabled (which enables MLDv2), on any interface it starts sending Query message, which is called general query to the all-hosts multicast group at ff02::1 periodically to discover whether any hosts want to receive multicast data.

OcNOS elects a router as the MLD querier on a subnet if it has the lowest IP address. As long as a router continues to receive query messages from a router with a lower IP address, it resets a timer that is based on its querier timeout value. If the querier timer of a router expires, it becomes the designated querier. If that router later receives a host query message from a router with a lower IP address, it drops its role as the designated querier and sets its querier timer again.

In the figure below Router-1 eth2 sends query every query-interval. Since Router1-eth2 IPv6 link local address is less than Router-2 eth2, Router-1 eth2 becomes querier on the LAN.

When a host receives a query from the local router it sends a Host Membership Report for all the multicast groups for which it wants to receive multicast traffic. This is called solicited membership report.

When a host joins a new group, the host immediately sends a Membership Report to inform a local router that it wants to receive multicast traffic for the group it has just joined without waiting to receive a Query. This is called unsolicited membership report.

In the figure below Host-1 and Host-2 sends membership reports to Router-1 eth2 for all the multicast groups for which they want to receive multicast traffic. Upon reception of membership report Router-1 maintains an MLD group table containing multicast group-address, interface name on which it receives the report.

When a multicast host leaves a group, a host that runs MLD sends an MLD leave message. To check if this host is the last host to leave the group, the router sends an MLD query (Called as Group-specific-query) message and starts a timer that you can configure called the last member query response interval. If no reports are received before the timer expires, the software removes the group state. The router continues to send multicast traffic for a group until its state is removed.

In the figure below Host-1 and Host-2 sends leave message to Router-1 eth2 for all the multicast groups for which they don't want to receive multicast traffic. In response to leave message Router-1 eth2 sends an group-specific-query message before removing the multicast group address from the MLD table.

The following example shows MLD configuration on Router1.

The configuration that follows shows how MLD version can be configured.

Enter the commands listed in this section to confirm the previous configurations.

The configuration that follows shows how MLD parameters can be configured.

Enter the commands listed in this section to confirm the previous configurations

MLD group table is populated at router by virtue of either static join is configured on interface or dynamic report is being received on the interface.

The show ipv6 mld groups command displays the MLD group table. In this table, the following fields are defined.