GE Reason Switches
Chapter 4
– Functions
REASON SWITCHES-TM-EN-3
73
Figure 35: IGMP protocol mechanism
When a
receiver wants to join an IP multicast group, it sends a “join group” message
to the network, where the routers will mark the incoming IP address and interface to
the group. After the receiver becomes a member of the desired multicast group, it
starts receiving data. It can be observed the sender will send data to a determined IP
address which will not be the address of the receivers, but a “virtual IP address” that
matches to the multicast IP address. To stop receiving data, the receiver sends a
“leave group” message to the routers at the network.
A range of the IP addresses which is used only for multicast is defined. When the
routers in the network receive a frame addressed to these IP addresses, they route
the frames based on multicast groups. Multicast IP addresses can be:
224.0.0.1
–
“All host” address
;
224.0.0.2
–
“All multicast routers” address
At the IGMP protocol, there are two addresses which are used by the protocol and
cannot be used as a multicast address:
From 224.0.0.0 to 239.255.255.255 (Class D address);
224.0.0.0 and 224.0.0.255 addresses are reserved for network protocols;
224.0.0.0 and 238.255.255.255 addresses are private addresses and cannot
be routed;
Reserved addresses cannot be used as IP multicast addresses. In addition to the addresses shown above, there are common
services, such as PTP multicasting and NTP multicasting, that have specific addresses. Be sure that they’re not used when
configuring an IP Multicast group at the network.
IGMP snooping function is performed by the switches by reading the IP header field
of the incoming packets. If the switch cannot handle IGMP snooping, multicast IP
communication is forwarded as broadcast transmission. Inspecting the IP header
data, at the port that is connected to layer 3 equipment (routers), the switch can
check if the packets are from IP multicast groups. If so, the switch can do a smart
forwarding decision, delivering the data only to the interfaces connected to the
multicast group, and save bandwidth in other Ethernet interfaces.
When there are many switches, such as LAN, the IGMP snooping will effectively save
bandwidth, as only the paths related to the multicast group will forward the data.
Paths between switches that are not member of the multicast group will not receive
its packets.
The figure below shows how IP multicast transmission happens through IGMP
snooping capable switches, routers and common switches. Orange lines mean traffic
through the path, and blue lines mean that there is no traffic at the path. It has been
considered all equipment is member of the same VLAN.
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