Multicast Listener Discovery (MLD) is a protocol used by IPv6 hosts to communicate their desire to receive multicast traffic to the neighboring multicast routers. It serves a role similar to that of Internet Group Management Protocol (IGMP) in IPv4 networks. MLD is essential for efficient multicast routing in IPv6 networks, ensuring that multicast data is only sent to network segments with interested receivers.

IP hosts use MLD to inform multicast routers about their membership in specific multicast groups, allowing routers to maintain a list of group memberships per interface. When a host joins a multicast group, it sends an MLD Report to the router, which updates its membership list. Routers then use this information to forward multicast data only to network segments with interested hosts, optimizing network resources by preventing unnecessary traffic.

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

MLD allows hosts to notify multicast routers about their interest in joining or leaving multicast groups, with routers maintaining membership lists for each interface. Hosts use MLD Report messages to join and Done messages to leave groups, enabling routers to update memberships. Routers then use this data to forward multicast traffic only to interested network segments, optimizing bandwidth. By default, MLDv2 is enabled with PIMv6, supporting source-specific multicast and maintaining compatibility with MLDv1. Administrators can manually configure MLD on interfaces as needed, ensuring effective multicast management and interoperability between versions.

These benefits make MLD an essential protocol for efficient and effective multicast routing in IPv6 networks, enhancing performance, scalability, and resource utilization.

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.

You can configure MLD on a network device to manage multicast group memberships effectively. This configuration enables efficient multicast traffic distribution, optimizes bandwidth usage, and ensures that multicast data is only sent to network segments with interested receivers.

This topology ensures that each router's interfaces are configured with the specified IP or IPv6 link-local addresses, and verifies the switch's configurations for connectivity. It involves setting up routing protocols or static routes on each router for communication, and assigning and configuring IPv6 addresses on router and host interfaces to ensure proper device communication via link-local addresses. Additionally, routers are configured to handle unicast or multicast traffic, with necessary multicast routing protocols set up for multicast traffic.

The network topology shown in the Figure 1-13 includes three routers, a switch, two hosts, and a source.

Configure Multicast Listener Discovery (MLD) on a network device. Activate MLD version 1 on the interface, which is responsible for managing the multicast group memberships.

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.

 

 

The following provides definitions for key terms or abbreviations and their meanings used throughout this document: