Using Multicast in Your Network
Fixed Switch Configuration Guide 19-3
Figure 19-1 IGMP Querier Determining Group Membership
As shown in Figure 19-1, a multicast-enabled device can periodically ask its hosts if they want to
receive multicast traffic. If there is more than one device on the LAN performing IP multicasting,
one of these devices is elected querier and assumes the responsibility of querying the LAN for
group members.
Based on the group membership information learned from IGMP, a device can determine which (if
any) multicast traffic needs to be forwarded to each of its ports. At Layer 3, multicast switch
devices use this information, along with a multicast routing protocol, to support IP multicasting
across the Internet.
IGMP provides the final step in IP multicast delivery. It is only concerned with forwarding
multicast traffic from the local switch device to group members on a directly attached subnetwork
or LAN segment.
IGMP neither alters nor routes any IP multicast packets. Since IGMP is not concerned with the
delivery of IP multicast packets across subnetworks, an external IP multicast device is needed if IP
multicast packets have to be routed across different subnetworks.
IGMP Support on Enterasys Devices
Enterasys devices implement IGMP version 2 (RFC 2236) and IGMP version 3 (RFC 3376), which
includes interoperability with version 1 hosts. IGMP version 1 is defined in RFC 1112.
Depending on your Enterasys device, IGMP can be configured independently at the switch level
(Layer 2) and at the router level (Layer 3).
Enterasys devices support IGMP as follows:
• Passively snooping on the IGMP query and IGMP report packets transferred between IP
multicast switches and IP multicast host groups to learn IP multicast group members. Each
Layer 2 device records which ports IGMP packets are received on, depending on the kind of
IGMP message, so multicast data traffic is not flooded across every port on the VLAN when it
is received by the switch.
IGMP Query
IGMP
Querier
Member of
224.1.1.1
Member of
226.7.8.9
IGMP Membership
Router for 226.7.8.9
IGMP Membership
Router for 224.1.1.1
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