Instruction Manual

Multimedia Traffic Control with IP Multicast (IGMP)
How IGMP Operates
Because of the multicast flooding problem mentioned above, the IGMP Fast-
Leave feature is disabled by default on all ProCurve switches that do not
support Data-Driven IGMP. (See the table above.) The feature can be enabled
on these switches via an SNMP set of this object:
hpSwitchIgmpPortForceLeaveState.<vid>.<port number>
However, this is not recommended as this will increase the amount of multi-
cast flooding during the period between the client’s IGMP Leave and the
Querier’s processing of that Leave. For more information on this topic refer
to “Forced Fast-Leave IGMP” on page page 2-17.
Automatic Fast-Leave Operation. If a switch port has the following char-
acteristics, then the Fast-Leave operation will apply:
1. Connected to only one end node
2. The end node currently belongs to a multicast group; i.e. is an IGMP client
3. The end node subsequently leaves the multicast group
Then the switch does not need to wait for the Querier status update interval,
but instead immediately removes the IGMP client from its IGMP table and
ceases transmitting IGMP traffic to the client. (If the switch detects multiple
end nodes on the port, automatic Fast-Leave does not activate—regardless of
whether one or more of these end nodes are IGMP clients.)
In the next figure, automatic Fast-Leave operates on the switch ports for IGMP
clients “3A” and “5A”, but not on the switch port for IGMP clients “7A” and 7B,
Server “7C”, and printer “7D”.
Routing
Switch
Acting as
Querier
Switch
Server
7C
Switch 7X
3A
5A
7A
Fast-Leave IGMP
automatically operates on
the ports connected to
IGMP clients 3A and 5A,
but does not operate on
the port connected to
Switch 7X because the
switch detects multiple
end nodes on that port.
Fast-Leave IGMP does
not activate on this port.
Fast-Leave IGMP
activates on these
two ports.
A1
A3
A4
A6
Printer
7D
7B
Figure 2-3. Example of Automatic Fast-Leave IGMP Criteria
When client “3A” running IGMP is ready to leave the multicast group, it
transmits a Leave Group message. Because the switch knows that there is only
one end node on port A3, it removes the client from its IGMP table and halts
multicast traffic (for that group) to port A3. If the switch is not the Querier, it
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