Layer 2 igmp (snooping and query), Igmp – Brocade Communications Systems Brocate Ethernet Access Switch 6910 User Manual
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Brocade 6910 Ethernet Access Switch Configuration Guide
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Layer 2 IGMP (Snooping and Query)
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This switch can use Internet Group Management Protocol (IGMP) to filter multicast traffic. IGMP
Snooping can be used to passively monitor or “snoop” on exchanges between attached hosts and
an IGMP-enabled device, most commonly a multicast router. In this way, the switch can discover
the ports that want to join a multicast group, and set its filters accordingly.
If there is no multicast router attached to the local subnet, multicast traffic and query messages
may not be received by the switch. In this case (Layer 2) IGMP Query can be used to actively ask the
attached hosts if they want to receive a specific multicast service. IGMP Query thereby 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.
The purpose of IP multicast filtering is to optimize a switched network’s performance, so multicast
packets will only be forwarded to those ports containing multicast group hosts or multicast
routers/switches, instead of flooding traffic to all ports in the subnet (VLAN).
You can also configure a single network-wide multicast VLAN shared by hosts residing in other
standard or private VLAN groups, preserving security and data isolation
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 IGMP Query (
) to monitor IGMP service requests
passing between multicast clients and servers, and dynamically configure the switch ports which
need to forward multicast traffic. IGMP Snooping conserves bandwidth on network segments
where no node has expressed interest in receiving a specific multicast service. For switches that do
not support multicast routing, or where multicast routing is already enabled on other switches in
the local network segment, IGMP Snooping is the only service required to support multicast
filtering.
When using IGMPv3 snooping, service requests from IGMP Version 1, 2 or 3 hosts are all
forwarded to the upstream router as IGMPv3 reports. The primary enhancement provided by
IGMPv3 snooping is in keeping track of information about the specific multicast sources which
downstream IGMPv3 hosts have requested or refused. The switch maintains information about
both multicast groups and channels, where a group indicates a multicast flow for which the hosts
have not requested a specific source (the only option for IGMPv1 and v2 hosts unless statically
configured on the switch), and a channel indicates a flow for which the hosts have requested
service from a specific source. For IGMPv1/v2 hosts, the source address of a channel is always null
(indicating that any source is acceptable), but for IGMPv3 hosts, it may include a specific address
when requested.
Only IGMPv3 hosts can request service from a specific multicast source. When downstream hosts
request service from a specific source for a multicast service, these sources are all placed in the
Include list, and traffic is forwarded to the hosts from each of these sources. IGMPv3 hosts may
also request that service be forwarded from any source except for those specified. In this case,
traffic is filtered from sources in the Exclude list, and forwarded from all other available sources.
NOTE
When the switch is configured to use IGMPv3 snooping, the snooping version may be downgraded
to version 2 or version 1, depending on the version of the IGMP query packets detected on each
VLAN.