Operation Manual

AT-RG213 Residential VoIP Gateway - SIP Software Reference Manual

SIP Software Release 6-0-0

J613-M0524-00

there is a process whereby all the routers back up the chain to the server

now know to forward the stream to this router – ie the router has joined onto

(‘grafted to’) the tree through which this stream is flowing.

IGMP snooping

IGMP snooping is something that layer-2 switches do to reduce the amount

of multicast traffic on a LAN.

IGMP snooping is a solution to a particular problem. To get an idea of what

the problem is, consider a LAN containing some layer-2 switches, and with a

router as a gateway (see Figure 6):

Figure 6. IGMP snooping network layers

If individual hosts on the LAN (ie host connected to ports on the switches)

wish to receive multicast streams, then they will send out IGMP joins, which

will get up to the router; and the router will join into the appropriate

multicast trees; and the multicast flows will then reach the router, and it will

forward them into the LAN. OK, so far so good.

But, lets think what happens when the multicast flows are forwarded into

the LAN. By default, when a switch receives a multicast packet, it must

forward it out all its ports (except the port upon which it was received). So,

if, say, ONLY host number 1 actually requests to join a particular multicast

group, what will happen is that ALL the hosts on the LAN will start

receiving the multicast packets, as all the switches will forward the multicast

packets to all their ports.