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Configuring MLD
This chapter describes MLD, how to MLD, configuration examples, and troubleshooting methods.
Overview
An IPv6 router uses the MLD protocol 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 that run MLD use an IPv6 unicast link-local address as the source address to send MLD
messages. MLD messages are ICMPv6 messages. All MLD messages are confined to the local
subnet, with a hop count of 1.
The term "interface" in this chapter collectively refers to
Layer 3 interfaces, including
VLAN interfaces
and Layer 3 Ethernet interfaces. You can set an Ethernet port as a Layer 3 interface by using the
port link-mode route
command (see
Layer 2—LAN Switching Configuration Guide
).
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 "
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. However, only one router can 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. Each router 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 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 considers
the querier has timed out and initiates a new querier election process.
