User guide

IGMP snooping Functional Overview (Includes New Functionality) IGMP snooping
3-3
iMG/RG Software Reference Manual (IGMP)
230.20.20.20 corresponds to MAC 01-00-5e-14-14-14
224.10.10.10 corresponds to MAC 01-00-5e-0a-0a-0a
Consequently, this is not a one-to-one mapping, but a one-to-many mapping:
224.10.10.10 corresponds to MAC 01-00-5e-0a-0a-0a
226.10.10.10 corresponds to MAC 01-00-5e-0a-0a-0a, as well.
It is required that when an IP multicast packet is sent onto an Ethernet, the destination MAC address of the
packet must be the MAC address that corresponds to the packet’s GDA. So, it is possible, from the destination
MAC address of a multicast packet, to know the set of values that its GDA must fall within.
3.1.2 IGMP snooping Functional Overview (Includes New Functionality)
IGMP snooping is a filtering process performed at layer 2 to reduce the amount of multicast traffic on a LAN.
It is designed to solve the problem when a multicast traffic is received from a layer 2 switch due to join requests
performed by hosts connected to some of the switch ports.
If individual hosts on the LAN (i.e. hosts connected to ports on the switches) wish to receive multicast streams,
then they will send out IGMP joins, which will get up to the multicast 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.
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, considering the example where only host number 1 actually requests to join a partic-
ular 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.
This is rather a waste of bandwidth, and the purpose of multicasting is to make efficient use of bandwidth.
The solution to this problem is to make the layer-2 switch aware of the IGMP packets that are being passed
around. That is, although the IGMP packets are destined for the router, the layer-2 switch needs to ‘snoop’ them
as they go past. Then the layer-2 switch can know which hosts have asked to join which multicast groups, and
only forward the multicast data to the places where it really needs to go.
Because the uplink interface can be connected to the network through an ADSL port, the igmp snooping fea-
ture is extended to include also the ADSL port when it is used on RFC1483 (bridged) connections.
IGMP snooping is designed to work in a network environment where both multicast router(s) and multicast
host(s) are present.
Note: Multicast packets having as destination IP the following range: 224.0.0.[0-255] and 224.0.1.[0-255] will
NOT be blocked in the upstream direction since belonging to reserved traffic (OSPF, RIPv2, PIM etc…)
The goal is to construct an internal view of the multicast network based on the IGMP messages received both
from multicast router(s) and multicast host(s).