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Multimedia Traffic Control with IP Multicast (IGMP)
How IGMP Operates
in the join request is determined by the requesting application running on the
IGMP client.) When a networking device with IGMP enabled receives the join
request for a specific group, it forwards any IP multicast traffic it receives for
that group through the port on which the join request was received. When the
client is ready to leave the multicast group, it sends a Leave Group message
to the network and ceases to be a group member. When the leave request is
detected, the appropriate IGMP device will cease transmitting traffic for the
designated multicast group through the port on which the leave request was
received (as long as there are no other current members of that group on the
affected port).
Thus, IGMP identifies members of a multicast group (within a subnet) and
allows IGMP-configured hosts (and routers) to join or leave multicast groups.
IGMP Data. To display data showing active group addresses, reports, que-
ries, querier access port, and active group address data (port, type, and
access), refer to the section titled “Internet Group Management Protocol
(IGMP) Status” in appendix B, “Monitoring and Analyzing Switch Operation”
of the Management and Configuration Guide for you switch.).
Operation With or Without IP Addressing
You can configure IGMP on VLANs that do not have IP addressing. The benefit
of IGMP without IP addressing is a reduction in the number of IP addresses
you have to use and configure. This can be significant in a network with a large
number of VLANs. The limitation on IGMP without IP addressing is that the
switch cannot become Querier on any VLANs for which it has no IP address—
so the network administrator must ensure that another IGMP device will act
as Querier. It is also advisable to have an additional IGMP device available as
a backup Querier. See the following table.
Table 2-1.Comparison of IGMP Operation With and Without IP Addressing
IGMP Function Available With IP Addressing
Configured on the VLAN
Available
Without IP
Addressing?
Operating Differences Without an IP Address
Forward multicast group traffic to any port on
the VLAN that has received a join request for
that multicast group.
Yes None
Forward join requests (reports) to the Querier. Yes None
Configure individual ports in the VLAN to Auto
(the default)/Blocked, or Forward.
Yes None
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