Understanding Mld Snooping - Cisco Catalyst 2960 series Configuration Manual

Information About Configuring IPv6 MLD Snooping
For complete syntax and usage information for the commands used in this chapter, see the command
Note
reference for this release or the Cisco IOS documentation referenced in the procedures.

Understanding MLD Snooping

In IP Version 4 (IPv4), Layer 2 switches can use Internet Group Management Protocol (IGMP) snooping to
limit the flooding of multicast traffic by dynamically configuring Layer 2 interfaces so that multicast traffic
is forwarded to only those interfaces associated with IP multicast devices. 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 by snooping
IPv6 multicast control packets.
MLD is a protocol used by IPv6 multicast routers to discover the presence of multicast listeners (nodes wishing
to receive IPv6 multicast packets) on the links that are directly attached to the routers 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,
identified in IPv6 packets by a preceding Next Header value of 58.
The switch supports two versions of MLD snooping:
• MLDv1 snooping detects MLDv1 control packets and sets up traffic bridging based on IPv6 destination
multicast addresses.
• MLDv2 basic snooping (MBSS) uses MLDv2 control packets to set up traffic forwarding based on IPv6
destination multicast addresses.
The switch can snoop on both MLDv1 and MLDv2 protocol packets and bridge IPv6 multicast data based on
destination IPv6 multicast addresses.
Note The switch does not support MLDv2 enhanced snooping, which sets up IPv6 source and destination
multicast address-based forwarding.
MLD snooping can be enabled or disabled globally or per VLAN. When MLD snooping is enabled, a per-VLAN
IPv6 multicast address table is constructed in software and hardware. The switch then performs IPv6
multicast-address based bridging in hardware.
MLD Messages
MLDv1 supports three types of messages:
• Listener Queries are the equivalent of IGMPv2 queries and are either General Queries or
Multicast-Address-Specific Queries (MASQs).
• Multicast Listener Reports are the equivalent of IGMPv2 reports
• Multicast Listener Done messages are the equivalent of IGMPv2 leave messages.
MLDv2 supports MLDv2 queries and reports, as well as MLDv1 Report and Done messages.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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