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138 Asanté IntraCore 36000 Series
Chapter 12: Layer 2 IGMP (Snooping and Query)
IGMP Snooping and Query – If multicast routing is not supported on other switches in your network, you can
use IGMP Snooping and Query (section
12.1.1 Configuring IGMP Snooping and Query Parameters ) to
monitor IGMP service requests passing between multicast clients and servers, and dynamically configure
the switch ports that need to forward multicast traffic.
Static IGMP Router Interface – If IGMP snooping cannot locate the IGMP querier, you can manually
designate a known IGMP querier (For example, a multicast router/switch) connected over the network to an
interface on your switch (section
12.1.4 ). This interface will then join all the current multicast groups
supported by the attached router/switch to ensure that multicast traffic is passed to all appropriate interfaces
within the switch.
Static IGMP Host Interface – For multicast applications that you need to control more carefully, you can
manually assign a multicast service to specific interfaces on the switch (section
12.1.5 ).
12.1 IGMP Multicast Filtering
Multicasting is used to support real-time applications such as videoconferencing or streaming audio. A
multicast server does not have to establish a separate connection with each client. It merely broadcasts its
service to the network, and any hosts that want to receive the multicast register with their local multicast
switch/router. Although this approach reduces the network overhead required by a multicast server, the
broadcast traffic must be carefully pruned at every multicast switch/router it passes through to ensure that
traffic is only passed on to the hosts that subscribed to this service.
This switch uses IGMP (Internet Group Management Protocol) to query for any attached hosts that want to
receive a specific multicast service. It identifies the ports containing hosts requesting to join the service and
sends data out to those ports only. It then propagates the service request up to any neighboring multicast
switch/router to ensure that it will continue to receive the multicast service. This procedure is called multicast
filtering.
12.1.1 Configuring IGMP Snooping and Query Parameters
You can configure the switch to forward multicast traffic intelligently. Based on the IGMP query and report
messages, the switch forwards traffic only to the ports that request multicast traffic. This prevents the switch
from broadcasting the traffic to all ports and possibly disrupting network performance.
Follow these guidelines:
•
IGMP Snooping – This switch can passively snoop on IGMP Query and Report packets transferred
between IP multicast routers/switches and IP multicast host groups to identify the IP multicast group
members. It simply monitors the IGMP packets passing through it, picks out the group registration
information, and configures the multicast filters accordingly.
•
IGMP Querier – A router, or multicast-enabled switch, can periodically ask their hosts if they want to
receive multicast traffic. If there is more than one router/switch on the LAN performing IP multicasting,
one of these devices is elected “querier” and assumes the role of querying the LAN for group members.
It then propagates the service requests on to any upstream multicast switch/router to ensure that it will
continue to receive the multicast service.
Note: Multicast routers use this information, along with a multicast routing protocol such as DVMRP or PIM,
to support IP multicasting across the Internet.
Command Attributes
IGMP Status — When enabled, the switch monitors network traffic to determine which hosts want to receive
multicast traffic. This is also referred to as IGMP Snooping. (Default: Enabled)