Quick Reference Guide

PowerConnect B-Series TI24X Configuration Guide 439
53-1002269-02
Chapter
18
Configuring IP Multicast Traffic Reduction for
PowerConnect B-Series TI24X Switches
IGMP snooping overview
When a device processes a multicast packet, by default, it broadcasts the packets to all ports
except the incoming port of a VLAN. Packets are flooded by hardware without going to the CPU. This
behavior causes some clients to receive unwanted traffic.
IGMP snooping provides multicast containment by forwarding traffic to only the ports that have
IGMP receivers for a specific multicast group (destination address). A device maintains the IGMP
group membership information by processing the IGMP reports and leave messages, so traffic can
be forwarded to ports receiving IGMP reports.
An IPv4 multicast address is a destination address in the range of 224.0.0.0 to 239.255.255.255.
Addresses of 224.0.0.X are reserved. Because packets destined for these addresses may require
VLAN flooding, devices do not snoop in the reserved range. Data packets destined to addresses in
the reserved range are flooded to the entire VLAN by hardware, and mirrored to the CPU. Multicast
data packets destined for the non-reserved range of addresses are snooped. A client must send
IGMP reports in order to receive traffic. If an application outside the reserved range requires VLAN
flooding, the user must configure a static group that applies to the entire VLAN.
An IGMP device's responsibility is to broadcast general queries periodically, and to send group
queries when receiving a leave message, to confirm that none of the clients on the port still want
specific traffic before removing the traffic from the port. IGMP V2 lets clients specify what group
(destination address) will receive the traffic but not to specify the source of the traffic. IGMP V3 is
for source-specific multicast traffic, adding the capability for clients to INCLUDE or EXCLUDE
specific traffic sources. An IGMP V3 device port state could be INCLUDE or EXCLUDE, and there are
different types of group records for client reports.
The interfaces respond to general or group queries by sending a membership report that contains
one or more of the following records associated with a specific group:
• Current-state record that indicates from which sources the interface wants to receive and not
receive traffic. This record contains the source address of interfaces and whether or not traffic
will be included (IS_IN) or not excluded (IS_EX) from this source.
• Filter-mode-change record. If the interface state changes from IS_IN to IS_EX, a TO_EX record
is included in the membership report. Likewise, if the interface state changes from IS_EX to
IS_IN, a TO_IN record appears in the membership report.
• An IGMP V2 leave report is equivalent to a TO_IN (empty) record in IGMP V3. This record means
that no traffic from this group will be received regardless of the source.
• An IGMP V2 group report is equivalent to an IS_EX (empty) record in IGMP V3. This record
means that all traffic from this group will be received regardless of source.
• Source-list-change record. If the interface wants to add or remove traffic sources from its
membership report, the report can contain an ALLOW record, which includes a list of new
sources from which the interface wishes to receive traffic. It can also contain a BLOCK record,
which lists the current traffic sources from which the interface wants to stop receiving traffic.