How Mld Works - IBM RackSwitch G8000 Application Manual

How MLD Works

© Copyright IBM Corp. 2011
The software uses the information obtained through MLD to maintain a list of
multicast group memberships for each interface and forwards the multicast traffic
only to interested listeners.
Without MLD, the switch forwards IPv6 multicast traffic through all ports, increasing
network load. Following is an overview of operations when MLD is configured on the
G8000:
• The switch acts as an Mrouter when MLDv1/v2 is configured and enabled on
each of its directly attached links. If the switch has multiple interfaces connected
to the same link, it operates the protocol on any one of the interfaces.
• If there are multiple Mrouters on the subnet, the Mrouter with the numerically
lowest IPv6 address is elected as the Querier.
• The Querier sends general queries at short intervals to learn multicast address
listener information from an attached link.
• Hosts respond to these queries by reporting their per-interface Multicast Address
Listening state, through Current State Report messages sent to a specific
multicast address that all MLD routers on the link listen to.
• If the listening state of a host changes, the host immediately reports these
changes through a State Change Report message.
• The Querier sends a Multicast Address Specific Query to verify if hosts are
listening to a specified multicast address or not. Similarly, if MLDv2 is configured,
the Querier sends a Multicast Address and Source Specific Query to verify, for a
specified multicast address, if hosts are listening to a specific set of sources, or
not. MLDv2 listener report messages consists of Multicast Address Records:
– INCLUDE: to receive packets from source specified in the MLDv2 message
– EXCLUDE: to receive packets from all sources except the ones specified in
the MLDv2 message
• A host can send a State Change Report to indicate its desire to stop listening to
a particular multicast address (or source in MLDv2). The Querier then sends a
multicast address specific query to verify if there are other listeners of the
multicast address. If there aren't any, the Mrouter deletes the multicast address
from its Multicast Address Listener state and stops sending multicast traffic.
Similarly in MLDv2, the Mrouter sends a Multicast Address and Source Specific
Query to verify if, for a specified multicast address, there are hosts still listening
to a specific set of sources.
G8000 supports MLD versions 1 and 2.
Note: MLDv2 operates in version 1 compatibility mode when, in a specific network,
not all hosts are configured with MLDv2.
Chapter 20. Multicast Listener Discovery
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