345
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 Any-Source Multicast (ASM) model. In addition, MLDv2 can directly
implement the Source-Specific Multicast (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 "
How MLDv1 works
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 only one router that acts as an MLD querier is needed for sending MLD query
messages (often called "queries"). A querier election mechanism selects 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. The destination address
is 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 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.