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MLD Snooping Overview 1
Forwarding mechanism in hardware
IP-based forwarding implementation on FCX and ICX devices
The following information about *,G or S,G fdb-based implementation is specific to FCX, ICX 6610,
ICX 6430, and ICX 6450 devices.
On both switch and router software images, MLD snooping is either *,G based or S,G based. The
hardware can either match the group address only (* G), or both the source and group (S, G) of the
data stream. The hardware can match only the lowest 32 bits of a 128 bit IPv6 address. This is
32-bit IP address matching, not 32-bit multicast MAC address 33-33-xx-xx-xx-xx matching.
If MLDv2 is configured in any port of a VLAN, the VLAN uses an (S, G) match, otherwise it uses (*
G). Because the hardware can match only the lowest 32 bits of a 128 bit IPv6 address, the output
interfaces (OIF) of a hardware resource are the superset of the OIF of all data streams sharing the
same lowest 32 bits. For example, if groups ff10::1234:5678:abcd and ff20::5678:abcd share the
same hardware resource, then the OIF of the hardware matching (* 5678:abcd) is the superset of
these two groups.
MAC-based forwarding implementation on FastIron X Series devices
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 33-33-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:40:2001:660:3007:123:0034:5678 maps to the IPv6 MAC address
33-33-00-34-56-78.
For two multicast traffic streams, Source_1 and Group1 (S1,G1) and Source_2 and Group2
(S2,G2), with the same or different source addresses, if the lowest 32 bits of the 128-bit IPv6
group address are the same, they would map to the same destination MAC. Because FSX devices
support MAC-based forwarding for MLD snooping, the final multicast MAC address entry would be a
superset of all the IPv6 groups mapped to it. For example, consider the following three IPv6
multicast streams sent from port 5 of a Brocade device:
• (S1,G1) = (2060::5, ff1e::12), client port 1, port 2
• (S2,G2) = (2060::6, ff1e:13::12), client port 2, port 3
• (S3,G1) = (2060::7, ff1e::12), client port 4
Because the lowest 32 bits of the group address for G1 and G2 are the same, all three streams
would use 33-33-00-00-00-12 as the destination MAC address. MLD snooping would build a MAC
entry with the MAC address 33-33-00-00-00-12 on egress ports 1, 2, 3, and 4. As a result, all three
streams would be sent to ports 1, 2, 3, and 4. Note that the above example assumes the following:
• The Brocade device is running MLD snooping on VLAN 10 and all three streams are in VLAN 10
• There are clients on port 1 and port 2 for (S1,G1)
• There are clients on port 2 and port 3 for (S2,G2)
• There are clients on port 4 for (S3,G1)
FastIron Ethernet Switch IP Multicast Configuration Guide 3
53-1002638-02