Configuring Multicast Forwarding
Multicast Forwarding
Cisco Small Business 300 Series Managed Switch Administration Guide 147
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For Multicast forwarding to work across IP subnets, nodes, and routers must be
Multicast-capable. A Multicast-capable node must be able to:
• Send and receive Multicast packets.
• Register the Multicast addresses being listened to by the node with local
routers, so that local and remote routers can route the multicast packet to
the nodes.
Typical Multicast Setup
While multicast routers route multicast packets between IP subnets, multicast-
capable Layer 2 switches forward multicast packets to registered nodes within a
LAN or VLAN.
A typical setup involves a router that forwards the Multicast streams between
private and/or public IP networks, a switch with Internet Group Membership
Protocol (IGMP) snooping capabilities, or Multicast Listener Discovery (MLD)
snooping, and a Multicast client that wants to receive a Multicast stream. In this
setup, the router sends IGMP queries periodically.
NOTE MLD for IPv6 is derived from the IGMP v2 for IPv4. Even though the description in
this section is mostly for IGMP, it also describes coverage of MLD where implied.
These queries reach the switch that in turn floods the queries to the VLAN, and
also learns the port where there is a Multicast router (Mrouter). When a host
receives the IGMP query message, it responds with an IGMP Join message saying
that the host wants to receive a specific Multicast stream and optionally from a
specific source. The switch with the IGMP snooping analyzes the Join messages,
and learns that the Multicast stream the host has requested must be forwarded to
this specific port. It then forwards the IGMP Join to the Mrouter only. Similarly,
when the Mrouter receives an IGMP Join message, it learns the interface where it
receives the Join messages wants to receive a specific multicast stream. The
Mrouter forwards the requested multicast stream to the interface.
Multicast Operation
In a Layer 2 Multicast service, a Layer 2 switch receives a single frame addressed
to a specific Multicast address. It creates copies of the frame to be transmitted on
each relevant port.
When the switch is IGMP/MLD snooping enabled and receives a frame for a
multicast stream, it forwards the multicast frame to all the ports that have
registered to receive the multicast stream using IGMP Join messages.
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