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ULTICAST
F
ILTERING
3-201
Layer 3 IGMP – This protocol includes a
form of multicast query specifically designed
to work with multicast routing. A router
periodically asks its hosts if they want to
receive multicast traffic. It then propagates
service requests on to any upstream multicast
router to ensure that it will continue to receive the multicast service. Layer
3 IGMP can be enabled for individual VLAN interfaces (page 3-200).
(Note that Layer 2 snooping and query is disabled if Layer 3 IGMP is
enabled.)
Configuring IGMP Interface Parameters
This switch uses IGMP (Internet Group Management Protocol) to query
for any attached hosts that want to receive a specific multicast service. The
hosts may respond with several types of IP multicast messages. Hosts
respond to queries with report messages that indicate which groups they
want to join or the groups to which they already belong. If a router does
not receive a report message within a specified period of time, it will prune
that interface from the multicast tree. A host can also submit a join
message at any time without waiting for a query from the router. Host can
also signal when they no longer want to receive traffic for a specific group
by sending a leave-group message.
These IGMP messages are used by the router to identify ports containing
multicast hosts and to restrict the downstream flow of multicast data to
only these ports. If more than one router on the LAN is performing IP
multicasting, one of these is elected as the “querier” and assumes the role
of querying for group members. It then propagates the service request up
to any neighboring multicast router to ensure that it will continue to
receive the multicast service. The following parameters are used to control
Layer 3 IGMP and query functions.
Command Attributes
• VLAN (Interface) – VLAN interface bound to a primary IP address.
L3 - network core
multicast routing
and L3 IGMP query
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