64 IGMP snooping
IGMP snooping
Feature overview A traditional Ethernet network may be separated into different network segments
to prevent placing too many devices onto the same shared media. Bridges and
switches connect these segments. When a packet with a broadcast or multicast
destination address is received, the switch will forward a copy into each of the
remaining network segments in accordance with the IEEE MAC Bridge standard.
Eventually, the packet is made accessible to all nodes connected to the network.
This approach works well for broadcast packets that are intended to be seen or
processed by all connected nodes. In the case of multicast packets, however, this
approach could lead to less efficient use of network bandwidth, particularly when
the packet is intended for only a small number of nodes.
IGMP snooping helps conserve switch bandwidth by preventing multicast traffic
from being forwarded on segments of the network where no node has expressed
interest in receiving packets addressed to group address. This contrasts with
normal switch behavior, where multicast traffic is typically forwarded on all
interfaces; that is, packets will be flooded into network segments where no node
has any interest in receiving the packet.
Enabling switches to snoop IGMP packets is a creative way to solve this
problem. The CN1610 switch uses the information in the IGMP packets as they
are being forwarded throughout the network to determine which segments should
receive packets directed to the group address. Multicast IP traffic is traffic that is
destined to a host group. Host groups are identified by class D IP addresses,
which range from 224.0.0.0 to 239.255.255.255. Based on the IGMP query and
report messages, the switch forwards traffic only to the ports that request the
multicast traffic. This prevents the switch from broadcasting the traffic to all
ports and possibly affecting network performance.
Implementation of
IGMP snooping
The IGMP snooping feature conforms with the IETF draft āConsiderations for
IGMP and MLD Snooping Switches,ā version 10, October 2003. The CN1610
switch implementation supports basic IGMPv3 functionality for processing
IGMPv3 join reports in the same manner in which IGMPv2 joins are processed;
that is, without giving consideration to the source address.
Operation The following steps explain the behavior of a multicast host, a switch and a
multicast router in a network segment where IGMP snooping is implemented:
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