Dell POWEREDGE M1000E - Manual (Page 437)

IGMP Snooping Commands 437
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IGMP Snooping Commands
Snooping of Internet Group Management Protocol (IGMP) messages is a
feature that allows PowerConnect switches to forward multicast traffic
intelligently on the switch. Multicast IP traffic is traffic that is destined to a
host group. Host groups are identified by class D IP addresses, which range
from 224.0.0.0 to 239.255.255.255. Based on the IGMP query and report
messages, the switch forwards traffic only to the ports that request the
multicast traffic. This prevents the switch from broadcasting the traffic to all
ports and possibly affecting network performance.
IGMP snooping switches build forwarding lists by monitoring for, and in
some cases intercepting, IGMP messages. Although the software processing
the IGMP messages could maintain state information based on the full IP
group addresses, the forwarding tables in PowerConnect are mapped to link
layer addresses.
The Multicast Forwarding Database (MFDB) manages the forwarding
address table for Layer 2 multicast protocols, such as IGMP Snooping.
The IGMP Snooping code in the CPU ages out IGMP entries in the MFDB.
If a report for a particular group on a particular interface is not received
within a certain time interval (query interval), the IGMP Snooping code
deletes that interface from the group. The value for query interval time is
configurable using management.
If an IGMP Leave Group message is received on an interface, the IGMP
Snooping code sends a query on that interface and waits a specified length of
time (maximum response time). If no response is received within that time,
that interface is removed from the group. The value for maximum response
time is configurable using management.
In addition to building and maintaining lists of multicast group
memberships, the snooping switch also maintains a list of multicast routers.
When forwarding multicast packets, they should be forwarded on ports that
have joined using IGMP and also on ports on which multicast routers are
attached. The reason for this is that in IGMP there is only one active query
mechanism. This means that all other routers on the network are suppressed
and thus not detectable by the switch. If a query is not received on an
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