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IGMP snooping overview
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, IGMP 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. This is 32-bit IP address matching, not 23-bit multicast MAC address
01-00-5e-xx-xx-xx matching.
When any port in a VLAN is configured for IGMP v3, the VLAN matches both source and group (S, G)
in hardware switching. If no ports are configured for IGMP v3, the VLAN matches group only (* G).
Matching (S, G) requires more hardware resources than matching (* G) when there are multiple
servers sharing the same group. For example, two data streams from different sources to the same
group require two (S, G) entries in IGMP v3, but only one (* G) entry in IGMP v2.
To conserve resources, IGMP v3 must be used only in source-specific applications. When VLANs
are independently configured for versions, some VLANs can match (* G) while others match (S, G).
MAC-based forwarding implementation on FastIron X Series devices
On both switch and router software images, IGMP snooping is MAC-based. This differs from IGMP
snooping on the BigIron router images, which match on both IP source and group (S,G) entries
programmed in the Layer 4 CAM.
This differs from IGMP snooping on the FastIron FCX/ICX router images, which match on both IP
source and group (S,G) entries. In contrast, the FastIron X Series images match on Layer 2 23-bit
multicast MAC address i.e. 01-00-5e-xx-xx-xx (*,G) entries.
In addition, the lowest 23 bits of the group address are mapped to a MAC address. In this way,
multiple groups (for example, 224.1.1.1 and 225.1.1.1) have the same MAC address. Groups
having the same MAC address are switched to the same destination ports, which are the superset
of individual group output ports. Thus, the use of Layer 2 CAM might cause unwanted packets to be
sent to some ports. However, the switch generally needs far less layer 2 mac entries than it does
for IP-based forwarding, which is required for each stream with a different source and group.
Hardware resources for IGMP/PIM-SM snooping
Brocade devices allocate/program fdb/mac entries and application VLAN (vidx) to achieve
multicast snooping in hardware. If a data packet does not match any of these resources, it might be
sent to the CPU, which increases the CPU burden. This can happen if the device runs out of
hardware resources, or is unable to install resources for a specific matching address due to a
hashing collision.
The hardware hashes addresses into available fdb/mac entries, with some addresses hashed into
the same entry. If the collision number in an entry is more than the hardware chain length, the
resource cannot be installed.
30 FastIron Ethernet Switch IP Multicast Configuration Guide
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