User manual
19
wEb-bASED bROwSER MANAGEMENT
A consumer may join any number of multicast groups, issuing a membership report
for each group. Hosts on the segment note membership reports from other hosts
and will suppress their own reports accordingly. In this way, the IGMP protocol
guarantees the segment will issue only one join for each group.
The router periodically queries each of its segments in order to determine if at least
one consumer still subscribes to a given stream. If no responses occur within a given
timeout period (usually two query intervals), the router will prune the multicast
stream from the given segment. A more usual method of pruning occurs when
consumers wishing to unsubscribe issue an IGMP “leave group” message. The
router will immediately issue a group-specic membership query to determine if
there are any remaining subscribers of that group on the segment. After the last
consumer of a group has un-subscribed, the router will prune the multicast stream
from the given segment.
IGMP Snooping Rules
When a multicast source starts multicasting, the trafc stream will be immediately
blocked on segments from which joins have not been received. The switch will
always forward all multicast trafc to the ports where multicast routers are attached
unless congured otherwise.
Packets with a destination IP multicast address in the 224.0.0.X range which are
not IGMP are always forwarded to all ports. This behavior is based on the fact that
many systems do not send joins for IP mulicast addresses in this range while still
listening to such packets. The switch will only send IGMP membership reports out
of those ports where multicast routers are attached because sending membership
reports to hosts could result in unintentionally preventing a host from joining a
specic group.
Multicast routers use IGMP to elect a master router known as the querier: the one
with the lowest IP address. All other routers become of non-queriers, participating