34 CHAPTER 3: USING MULTICAST FILTERING
routers on the subnetwork. Upon receiving such a message, the querier
determines whether that host is the last group member on the
subnetwork by issuing a group-specific query.
Leave-group messages reduce leave latency — that is, the time between
when the last group member on a given subnetwork sends a report and
when a router stops forwarding traffic for that group onto the
subnetwork. This process conserves bandwidth. The alternative is for the
router to wait for at least two queries to go unanswered before pruning
that subnetwork from the delivery tree.
Role of IGMP in IP
Multicast Filtering
To further refine the IP multicast delivery process and maximize
bandwidth efficiency, the Switch filters IP multicast packets on
appropriate ports using a process called IGMP snooping. Both bridged
interfaces and routed interfaces record which ports receive host IGMP
reports and then set their filters accordingly so that IP multicast traffic for
particular groups is not forwarded on ports or VLANs that do not require
it.
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