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Multimedia Traffic Control with IP Multicast (IGMP)
How IGMP Operates
Automatic Fast-Leave IGMP
IGMP Operation Presents a “Delayed Leave” Problem. Where multiple
IGMP clients are connected to the same port on an IGMP device (switch or
router), if only one IGMP client joins a given multicast group, then later sends
a Leave Group message and ceases to belong to that group, the IGMP device
retains that IGMP client in its IGMP table and continues forwarding IGMP
traffic to the IGMP client until the Querier triggers confirmation that no other
group members exist on the same port. This means that the switch continues
to transmit unnecessary multicast traffic through the port until the Querier
renews multicast group status.
Fast-Leave IGMP Reduces Leave Delays. Fast-Leave IGMP automatically
operates on a port if an IGMP client connects to the port and there are no other
end nodes detected on that port. In this case, when the client leaves a multicast
group, Fast-Leave IGMP automatically accelerates the blocking of further,
unnecessary multicast traffic from that group to the former IGMP client. This
improves performance by reducing the amount of multicast traffic going
through the port to the IGMP client after the client leaves a multicast group.
Automatic Fast-Leave Operation. If a switch port is:
a. Connected to only one end node
b. The end node currently belongs to a multicast group; i.e. is an IGMP
client
c. 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.)
!Software.book Page 13 Thursday, October 10, 2002 6:10 PM
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