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Intelligent multicasting refers to the forwarding of multicast traffic (packets with a multicast destination MAC address) within a VLAN to a subset of ports participating in that VLAN. It limits the forwarding of multicast traffic to only those ports on the VLAN with clients that want to receive this multicast traffic.
When disabled, intelligent multicasting floods layer 2 multicast traffic to all ports on the VLAN on which the traffic is received.
All traffic that is sent to a particular multicast MAC address is said to be in a multicast session. The switch supports 58 sessions per VLAN. Each multicast session keeps track of which ports must receive that session's multicast traffic within the VLAN. There are two types of ports: client ports and router ports.
A client port is a port with an attached host configured to receive a multicast session.
Router ports are ports that are attached to (or in the path to) multicast routers and must be treated specially. All multicast traffic on a VLAN must be forwarded to the router port.
Configuration of an Intelligent Multicast session first requires a session to be established. Once that session is established, client and router ports can be added to or removed from the session. Session and port configuration can be done either manually or dynamically. Dynamic intelligent multicasting is achieved through Internet Group Management Protocol (IGMP) Snooping, and may also involve Lucent Group Membership Protocol (LGMP), or Cisco Group Management Protocol (CGMP) Snooping. All of the dynamic mechanisms are based on the assumption that the client host is running IGMP, and is requesting membership in the IP multicast session.
Note: If there is no multicast session created for a multicast flow in a VLAN, then that multicast flow will be flooded to all ports on the VLAN. This is the default behavior for a bridge as described in IEEE 802.1D. Intelligent multicasting must be enabled for any dynamic intelligent multicasting to be active.
By default, the switch rate limits inter-router multicast traffic on all modules that support rate limiting. If you need for your switch to support heavy multicast traffic, disable rate limiting on ports that are connected to routers. Also note that if high-bandwidth multicast streams are being used, rate limiting will affect directly connected clients if that stream is not part of an Intelligent Multicast session.
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