H3C Technologies H3C S12500 Series Switches - Manual (Page 357)

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Configuring MLD
Overview
The Multicast Listener Discovery protocol (MLD) is used by an IPv6 router to discover the presence of
multicast listeners on the directly attached subnets. Multicast listeners are nodes wishing to receive IPv6
multicast packets.
Through MLD, the router can learn whether any IPv6 multicast listeners exist on the directly connected
subnets, put corresponding records in the database, and maintain timers related to IPv6 multicast
addresses.
Routers running MLD use an IPv6 unicast link-local address as the source address to send MLD messages.
MLD messages are Internet Control Message Protocol for IPv6 (ICMPv6) messages. All MLD messages
are confined to the local subnet, with a hop count of 1.
The term "router" in this document refers to both routers and Layer 3 switches.
MLD versions
• MLDv1 (defined in RFC 2710), which is derived from IGMPv2.
• MLDv2 (defined in RFC 3810), which is derived from IGMPv3.
All MLD versions support the ASM model. In addition, MLDv2 can directly implement the SSM model, but
MLDv1 must work with the MLD SSM mapping function to implement SSM service. For more information
about the ASM and SSM models, see "Multicast overview."
How MLDv1 operates
MLDv1 implements IPv6 multicast listener management based on the query/response mechanism.
MLD querier election
All IPv6 multicast routers on the same subnet can monitor MLD listener report messages (often called
"reports") from hosts, but the subnet needs only one router to act as the MLD querier to send MLD query
messages (often called "queries"). A querier election mechanism determines which router will act as the
MLD querier on the subnet.
1. Initially, every MLD router assumes itself as the querier and sends MLD general query messages
(often called "general queries") to all hosts and routers on the local subnet with destination address
of FF02::1.
2. After receiving a general query, every MLD router compares the source IPv6 address of the query
message with its own interface address. After comparison, the router with the lowest IPv6 address
wins the querier election and becomes the querier, and all other routers become non-queriers.
3. All the non-queriers start a timer, called the "other querier present timer." If a router receives an
MLD query from the querier before the timer expires, it resets this timer. Otherwise, it assumes that
the querier has timed out and initiates a new querier election process.