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2<br />
IGMP snooping overview<br />
An IPv4 multicast address is a destination address in the range of 224.0.0.0 to 239.255.255.255.<br />
Addresses of 224.0.0.X are reserved. Because packets destined for these addresses may require<br />
VLAN flooding, devices do not snoop in the reserved range. Data packets destined to addresses in<br />
the reserved range are flooded to the entire VLAN by hardware, and mirrored to the CPU. Multicast<br />
data packets destined for the non-reserved range of addresses are snooped. A client must send<br />
IGMP reports in order to receive traffic.<br />
An IGMP device's responsibility is to broadcast general queries periodically, and to send group<br />
queries when receiving a leave message, to confirm that none of the clients on the port still want<br />
specific traffic before removing the traffic from the port. IGMP V2 lets clients specify what group<br />
(destination address) will receive the traffic but not to specify the source of the traffic. IGMP V3 is<br />
for source-specific multicast traffic, adding the capability for clients to INCLUDE or EXCLUDE<br />
specific traffic sources. An IGMP V3 device port state could be INCLUDE or EXCLUDE, and there are<br />
different types of group records for client reports.<br />
The interfaces respond to general or group queries by sending a membership report that contains<br />
one or more of the following records associated with a specific group:<br />
• Current-state record that indicates from which sources the interface wants to receive and not<br />
receive traffic. This record contains the source address of interfaces and whether or not traffic<br />
will be included (IS_IN) or not excluded (IS_EX) from this source.<br />
• Filter-mode-change record. If the interface state changes from IS_IN to IS_EX, a TO_EX record<br />
is included in the membership report. Likewise, if the interface state changes from IS_EX to<br />
IS_IN, a TO_IN record appears in the membership report.<br />
• An IGMP V2 leave report is equivalent to a TO_IN (empty) record in IGMP V3. This record means<br />
that no traffic from this group will be received regardless of the source.<br />
• An IGMP V2 group report is equivalent to an IS_EX (empty) record in IGMP V3. This record<br />
means that all traffic from this group will be received regardless of source.<br />
• Source-list-change record. If the interface wants to add or remove traffic sources from its<br />
membership report, the report can contain an ALLOW record, which includes a list of new<br />
sources from which the interface wishes to receive traffic. It can also contain a BLOCK record,<br />
which lists the current traffic sources from which the interface wants to stop receiving traffic.<br />
IGMP protocols provide a method for clients and a device to exchange messages, and let the device<br />
build a database indicating which port wants what traffic. The protocols do not specify forwarding<br />
methods. They require IGMP snooping or multicast protocols such as PIM to handle packet<br />
forwarding. PIM can route multicast packets within and outside a VLAN, while IGMP snooping can<br />
switch packets only within a VLAN.<br />
If a VLAN is not IGMP snooping-enabled, it floods multicast data and control packets to the entire<br />
VLAN in hardware. When snooping is enabled, IGMP packets are trapped to the CPU. Data packets<br />
are mirrored to the CPU in addition to being VLAN flooded. The CPU then installs hardware<br />
resources, so that subsequent data packets can be switched to desired ports in hardware without<br />
going to the CPU. If there is no client report or port to queriers for a data stream, the hardware<br />
resource drops it.<br />
Queriers and non-queriers<br />
An IGMP snooping-enabled <strong>Brocade</strong> device can be configured as a querier (active) or non-querier<br />
(passive). An IGMP querier sends queries; a non-querier listens for IGMP queries and forwards<br />
them to the entire VLAN. VLANs can be independently configured to be queriers or non-queriers. If<br />
a VLAN has a connection to a PIM-enabled port on another router, the VLAN must be configured as<br />
a non-querier. When multiple IGMP snooping devices are connected together, and there is no<br />
28 FastIron Ethernet Switch IP Multicast Configuration Guide<br />
53-1002638-02
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