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MLD Snooping Overview
MLD protocols provide a way for clients and a device to exchange messages, and allow the device
to build a database indicating which port wants what traffic. Since the MLD protocols do not specify
forwarding methods, MLD snooping or multicast protocols such as IPv6 PIM-Sparse Mode (PIM6
SM) are required to handle packet forwarding. PIM6 SM can route multicast packets within and
outside a VLAN, while MLD snooping can switch packets only within a VLAN.
MLD snooping provides multicast containment by forwarding traffic only to those clients that have
MLD receivers for a specific multicast group (destination address). The device maintains the MLD
group membership information by processing MLD reports and generating messages so traffic can
be forwarded to ports receiving MLD reports. This is analogous to IGMP Snooping on Brocade Layer
3 switches.
An IPv6 multicast address is a destination address in the range of FF00::/8. A limited number of
multicast addresses are reserved. Because packets destined for the reserved addresses may
require VLAN flooding, FSX devices do not snoop in the FF0X::00X range (where X is from 00 to FF)
and FFXX:XXXX:XXXX:XXXX:XXXX:XXXX:1:2. Data packets destined to these addresses are flooded to
the entire VLAN by hardware and mirrored to the CPU. Multicast data packets destined to
addresses outside the FF0X::00X range and FFXX:XXXX:XXXX:XXXX:XXXX:XXXX:1:2 are snooped. A
client must send MLD reports in order to receive traffic.
An MLD device periodically sends general queries and sends group queries upon receiving a leave
message, to ensure no other clients at the same port still want this specific traffic before removing
it. MLDv1 allows clients to specify which group (destination IPv6 address) will receive traffic.
(MLDv1 cannot choose the source of the traffic.) MLDv2 deals with source-specific multicasts,
adding the capability for clients to INCLUDE or EXCLUDE specific traffic sources. An MLDv2 device's
port state can either be in INCLUDE or EXCLUDE mode.
There are different types of group records for client reports. Clients respond to general queries by
sending a membership report containing one or more of the following records associated with a
specific group:
• Current-state record - Indicates the sources from which the client wants to receive or not
receive traffic. This record contains the addresses of the multicast sources and indicates
whether or not traffic will be included (IS_IN) or excluded (IS_EX) from that source address.
• Filter-mode-change record - If the client changes its current state from IS_IN to IS_EX, a TO_EX
record is included in the membership report. Likewise, if a client current state changes from
IS_EX to IS_IN, a TO_IN record appears in the membership report.
• MLDv1 leave report - Equivalent to a TO_IN (empty) record in MLDv2. This record means that
no traffic from this group will be received, regardless of the source.
• An MLDv1 group report - Equivalent to an IS_EX (empty) record in MLDv2. This record means
that all traffic from this group will be received, regardless of the source.
• Source-list-change record - If the client wants to add or remove traffic sources from its
membership report, the report can include an ALLOW record, which contains a list of new
sources from which the client wishes to receive traffic. The report can also contain a BLOCK
record, which lists current traffic sources from which the client wants to stop receiving traffic.
Support for MLD snooping and Layer 3 v6 multicast routing together on
the same device
The Brocade device supports global Layer 2 IPv6 multicast traffic reduction (MLD snooping) and
Layer 3 v6 multicast routing (PIM-Sparse) together on the same device in the full Layer 3 software
image, as long as the Layer 2 feature configuration is at the VLAN level.
2 FastIron Ethernet Switch IP Multicast Configuration Guide
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