Configuring L2 Multicast Features 699
these IGMP reports to identify multicast router ports. If there is another
querier in the network and the local querier is in election mode, then the
querier with the lower IP address is elected and the other querier stops
querying. If the local querier is not in election mode and another querier is
detected, the local querier stops querying.
What Is MLD Snooping?
In IPv4, Layer 2 switches can use IGMP snooping to limit the flooding of
multicast traffic by dynamically configuring the multicast forwarding
database so that multicast data traffic is forwarded to only those ports
associated with a multicast router or host that has indicated an interest in
receiving a particular multicast group. In IPv6, MLD snooping performs a
similar function.
With MLD snooping, IPv6 multicast data is selectively forwarded to a list of
ports that want to receive the data instead of being flooded to all ports in a
VLAN. This list is constructed in the MFDB by snooping IPv6 multicast
control packets. MLD snooping floods multicast data packets until a
multicast router port has been identified. MLD snooping forwards
unregistered multicast data packets to IPv6 multicast routers. MLD snooping
discovers multicast routers by listening for MLD queries and populates the
MFDB.
MLD is a protocol used by IPv6 multicast routers to discover the presence of
multicast listeners (nodes wishing to receive IPv6 multicast packets) on its
directly-attached links and to discover which multicast packets are of interest
to neighboring nodes. MLD is derived from IGMP; MLD version 1 (MLDv1)
is equivalent to IGMPv2, and MLD version 2 (MLDv2) is equivalent to
IGMPv3. MLD is a subprotocol of Internet Control Message Protocol version
6 (ICMPv6), and MLD messages are a subset of ICMPv6 messages.
The switch snoops both MLDv1 and MLDv2 protocol packets and forwards
IPv6 multicast data based on destination IPv6 multicast MAC addresses
(33:33::). The switch floods multicast control plane traffic addressed to the
permanently assigned (well-known) multicast address FF0x::/12 to all ports in
the VLAN, except for MLD packets, which are handled according the MLD
snooping rules.
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