The device supports MLDv1 and MLDv2. MLDv2 supports MLDv1 listener reports.
By default, the software enables MLDv2 when it starts the MLD process. You can enable MLDv1 on interfaces where you want only its capabilities.
MLDv2 includes the following key changes from MLDv1:
- Support for Source-Specific Multicast (SSM), which builds shortest path trees from each receiver to the source, through the following features:
– Host messages that can specify both the group and the source.
– The multicast state that is maintained for groups and sources, not just for groups as in MLDv1.
- Hosts no longer perform report suppression, which means that hosts always send MLD listener reports when an MLD query message is received.
For detailed information about MLDv1, see RFC 2710. For detailed information about MLDv2, see RFC 3810.
The basic MLD process of a router that discovers multicast hosts is shown in Figure 1-1. Hosts 1, 2, and 3 send unsolicited MLD listener report messages to initiate receiving multicast data for a group or channel.
Figure 1-1 MLD Query-Response Process
In Figure 1-1, router A, which is the MLD designated querier on the subnet, sends a general query message to the link-scope all-nodes multicast address FF02::1 periodically to discover what multicast groups hosts want to receive. The group-specific query is used to discover whether a specific group is requested by any hosts. You can configure the group membership timeout value that the router uses to determine that no members of a group or source exist on the subnet. For more information about configuring the MLD parameters, see the “Configuring MLD Interface Parameters” section.
In Figure 1-1, host 1’s listener report is suppressed, and host 2 sends its listener report for group FFFE:FFFF:90::1 first. Host 1 receives the report from host 2. Because only one listener report per group needs to be sent to the router, other hosts suppress their reports to reduce network traffic. Each host waits for a random time interval to avoid sending reports at the same time. You can configure the query maximum response time parameter to control the interval in which hosts randomize their responses.
Note MLDv1 membership report suppression occurs only on hosts that are connected to the same port.
In Figure 1-2, router A sends the MLDv2 group-and-source-specific query to the LAN. Hosts 2 and 3 respond to the query with listener reports that indicate that they want to receive data from the advertised group and source. This MLDv2 feature supports SSM. For information about configuring SSM translation to support SSM for MLDv1 hosts, see the “Configuring an MLD SSM Translation” section.
Note In MLDv2, all hosts respond to queries.
Figure 1-2 MLDv2 Group-and-Source-Specific Query
The software 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 remains a nonquerier and 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.
Messages sent by the designated querier have a time-to-live (TTL) value of 1, which means that the messages are not forwarded by the directly connected routers on the subnet, and you can configure the frequency and number of query messages sent specifically for MLD startup. You can configure a short query interval at startup so that the group state is established as quickly as possible. Although usually unnecessary, you can tune the query interval used after startup to a value that balances responsiveness to host group membership and the traffic created on the network.
If you change the query interval, you can severely impact multicast forwarding in your network.
When a multicast host leaves a group, it should send a done message for MLDv1, or a listener report that excludes the group to the link-scope all-routers multicast address FF02::2. To check if this host is the last host to leave the group, the software sends an MLD 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.
You can configure a robustness value to compensate for the packet loss on a congested network. The robustness value is used by the MLD software to determine the number of times to send messages.
Link local addresses in the range FF02::0/16 have link scope, as defined by the Internet Assigned Numbers Authority (IANA). Network protocols on a local network segment use these addresses; routers do not forward these addresses because they have a TTL of 1. By default, the MLD process sends listener reports only for nonlink local addresses, but you can configure the software to send reports for link local addresses.
For more information about configuring the MLD parameters, see the “Configuring MLD Interface Parameters” section.
A virtual device context (VDC) is a logical representation of a set of system resources. Within each VDC, you can define multiple virtual routing and forwarding (VRF) instances. One MLD process can run per VDC. The MLD process supports all VRFs in that VDC.
You can use the show commands with a VRF argument to provide a context for the information displayed. The default VRF is used if no VRF argument is supplied.
For information about configuring VDCs, see the Cisco Nexus 7000 Series NX-OS Virtual Device Context Configuration Guide, Release 4.2.
For information about configuring VRFs, see the Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 5.x.