IGMP Snooping Enabled Introduction
5-3
Software Reference for SwitchBlade x3100 Series Switches (Internet Group Management Pro-
tocol (IGMP))
5.2.2 IGMP Snooping Enabled
As mentioned above, IGMP snooping allows the SBx3112 to conserve local area network bandwidth by not
broadcasting a received multicast frame but rather forwarding the multicast frame only to those ports that have
expressed an interest in receiving such frames. (The default forwarding behavior of a LAN switch for unicast
and broadcast frames is not affected.) The snooping will configure the hardware to allow multicast streams for
that group to be forwarded only to ports that have requested that stream.
The SBx3112 keeps track of Multicast Group membership by MAC address, so Leaves are processed immedi-
ately and the interface is removed from the Multicast Group (no timers). This is called IGMP Fast Leave.
The parameter
DUPREPORTTIMER (of the SET IGMPSNOOPING command) helps control the number of dupli-
cate reports sent to the multicast router(s). Also, the SBx3112 only sends up the last Leave to the router.
Once IGMP has been enabled, the system may generate a warning message at the user’s CLI session stating that
classifier capacity or capabilities have been exceeded on the slot(s) impacted by the provisioning change. The
user should investigate classifier-related provisioning, such as IGMP, DHCPRELAY, VLAN (for per-VLAN UFO
and HVLAN), EPSR, INTERFACE (TAGALL option for HVLAN), ACCESSLIST, and CLASSIFIER to determine
the reason for the message.
5.2.3 IGMP Snooping at the System and Interface Level
Since the IGMP Snooping feature can be enabled and disabled at both the system and port level, the following
interactions apply:
• If IGMP Snooping is disabled system wide, all multicast packets will be flooded (within the VLAN) including
IGMP control packets (Reports, General Queries, Groups Specific Queries). In this case, no IGMP control
messages are forwarded to the CPU for processing; they are just switched (flooded).
• If IGMP Snooping is enabled system wide and the port level control is enabled, then the port is snooped;
IGMP Snooping software in the CFC will receive Reports and Leaves and process them as follows:
• Unrequested (no Report processed by IGMP Snooping) multicast packets are dropped at the CFC
switch.
• Requested multicast packets are only sent to the ports where an IGMP Report is received.
• If IGMP Snooping is enabled system wide and the port level control is disabled, then any IGMP Control
Packet (Reports and Leaves) that are received from that port will be discarded (not processed and not
flooded). This port will not be able to participate in IGMP.
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