Note
Changing the query interval can severely impact multicast forwarding.
When a multicast host leaves a group, a host that runs IGMPv2 or later sends an IGMP leave message. To check if this host is the last host to leave the group, the software sends an IGMP 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 packet loss on a congested network. The robustness value is used by the IGMP software to determine the number of times to send messages.
Link-local addresses in the range 224.0.0.0/24 are reserved 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 IGMP process sends membership reports only for nonlink-local addresses, but you can configure the software to send reports for link-local addresses.
IGMP snooping is a feature that limits multicast traffic on VLANs to the subset of ports that have known receivers. The IGMP snooping software examines IGMP protocol messages within a VLAN to discover which interfaces are connected to hosts or other devices interested in receiving this traffic. Using the interface information, IGMP snooping can reduce bandwidth consumption in a multi-access LAN environment to avoid flooding the entire VLAN. The IGMP snooping feature tracks which ports are attached to multicast-capable routers to help it manage the forwarding of IGMP membership reports. Multicast traffic is sent only to VLAN ports on which interested hosts reside. The IGMP snooping software responds to topology change notifications.
By default, IGMP snooping is enabled on the Cisco NX-OS system.
Multicast Listener Discovery (MLD) is an IPv6 protocol that a host uses to request multicast data for a particular group. Using the information obtained through MLD, the software maintains a list of multicast group or channel memberships on a per-interface basis. The devices that receive MLD packets send the multicast data that they receive for requested groups or channels out the network segment of the known receivers.
MLDv1 is derived from IGMPv2, and MLDv2 is derived from IGMPv3. IGMP uses IP Protocol 2 message types, whereas MLD uses IP Protocol 58 message types, which is a subset of the ICMPv6 messages.
The MLD process is started automatically on the device. You cannot enable MLD manually on an interface. MLD is enabled automatically when you perform one of the following configuration tasks on an interface:
Enable PIM6.
Statically bind a local multicast group.
Enable link-local group reports.
Cisco NX-OS 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:
MLDv2 supports 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 MLD process is similar to IGMP: MLD utilizes link-local addresses in the range FF02::0/16, as defined by the 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.
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