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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring IPv6 MLD Snooping
Understanding MLD Snooping
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 (MESS), 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 MAC address table is constructed in software and a per-VLAN IPv6 multicast
address table is constructed in software and hardware. The switch then performs IPv6 multicast-address
based bridging in hardware.
According to IPv6 multicast standards, the switch derives the MAC multicast address by performing a
logical-OR of the four low-order octets of the switch MAC address with the MAC address of
33:33:00:00:00:00. For example, the IPv6 MAC address of FF02:DEAD:BEEF:1:3 maps to the Ethernet
MAC address of 33:33:00:01:00:03.
A multicast packet is unmatched when the destination IPv6 address does not match the destination MAC
address. The switch forwards the unmatched packet in hardware based the MAC address table. If the
destination MAC address is not in the MAC address table, the switch floods the packet to all ports in the
same VLAN as the receiving port.
These sections describe some parameters of IPv6 MLD snooping:
• MLD Messages, page 1-3
• MLD Queries, page 1-3
• Multicast Client Aging Robustness, page 1-3
• Multicast Router Discovery, page 1-4
• MLD Reports, page 1-4
• MLD Done Messages and Immediate-Leave, page 1-4
• Topology Change Notification Processing, page 1-5
• MLD Snooping in Switch Stacks, page 1-5
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