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
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
On both switch and router software images, IGMP snooping is
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
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 and 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.
Configuration notes and feature limitations for IGMP snooping and Layer 3 multicast routing
The following notes apply to all devices:
•Layer 2 IGMP multicast is automatically enabled with Layer 3 multicast routing. If Layer 3 multicast routing is enabled on your system, do not attempt to enable Layer 2 IGMP snooping.
•The default IGMP version is V2.
•A user can configure the maximum numbers of group address entries.
•An IGMP device can be configured to
•The device supports static groups. The device acts as a proxy to send IGMP reports for the static groups when receiving queries.
16 | FastIron Ethernet Switch IP Multicast Configuration Guide |
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