Chapter 9 - Multicast - Linksys LGS308 User Manual

Chapter 9 – Multicast
Multicast forwarding enables one-to-many information dissemination. Multicast applications are
useful for dissemination of information to multiple clients, where clients do not require reception
of the entire content. A typical application is a cable-TV-like service, where clients can join a
channel in the middle of a transmission, and leave before it ends.
The data is sent only to relevant ports. Forwarding the data only to the relevant ports conserves
bandwidth and host resources on links.
For Multicast forwarding to work across IP subnets, nodes and routers must be Multicast-
capable. A Multicast-capable node must be able to do the following:
Send and receive Multicast packets.
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• 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 device with Internet Group Membership Protocol (IGMP) snooping
capabilities, and a Multicast client that wants to receive a Multicast stream. In this setup, the
router sends IGMP queries periodically.
These queries reach the device, which 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 device with 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 from which it received the Join messages
that wants to receive a specific Multicast stream. The Mrouter forwards the requested Multicast
stream to the interface.
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 device is IGMP 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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