Configuration Guide User guide

1612 FastIron Configuration Guide
53-1002494-02
MLD Snooping Overview
An IPv6 multicast address is a destination address in the range of FF00::/8. A limited number of
multicast addresses are reserved. Since packets destined for the reserved addresses may require
VLAN flooding, FESX and FSX devices do not snoop in the FF0X::00X range (where X is from 00 to
FF). 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
are snooped. A client must send MLD reports in order to receive traffic.
An MLD device periodically broadcasts 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 source addresses of the clients 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.
How MLD snooping uses MAC addresses to forward
multicast packets
Multicast Listening Discovery (MLD) snooping on Brocade devices is based on MAC address
entries. When an IPv6 multicast data packet is received, the packet destination MAC is matched
with the MAC address entries in the IPv6 multicast table. If a match is found, packets are sent to
the ports associated with the MAC address. If a match is not found, packets are flooded to the
VLAN and copied to the CPU.
For IPv6 multicast, the destination MAC address is in the format 0x33-33-xx-yy-zz-kk, where
xx-yy-zz-kk are the 32 lowest bits of the IPv6 multicast group address. For example, the IPv6 group
address 0xFF3E:40:2001:660:3007:123:1234:5678 maps to the IPv6 MAC address
0x33-33-12-34-56-78.