Specifications

68 Implementing IBM System Networking 10Gb Ethernet Switches
IGMP Snooping conserves bandwidth. With IGMP Snooping, the switch learns which ports
are interested in receiving multicast data, and forwards multicast data only to those ports. In
this way, other ports are not burdened with unwanted multicast traffic.
The switch can sense IGMP Membership Reports from attached clients and act as a proxy to
set up a dedicated path between the requesting host and a local IPv4 Multicast router. After
the pathway is established, the switch blocks the IPv4 Multicast stream from flowing through
any port that does not connect to a host member, thus conserving bandwidth.
The client-server path is set up as follows:
1. An IPv4 Multicast Router (Mrouter) sends Membership Queries to the switch, which
forwards them to all ports in a VLAN.
2. Hosts that want to receive the multicast data stream send Membership Reports to the
switch, which sends a proxy Membership Report to the Mrouter.
3. The switch sets up a path between the Mrouter and the host, and blocks all other ports
from receiving the multicast.
4. Periodically, the Mrouter sends Membership Queries to ensure that the host wants to
continue receiving the multicast. If a host fails to respond with a Membership Report, the
Mrouter stops sending the multicast to that path.
5. The host can send a Leave Report to the switch, which sends a proxy Leave Report to the
Mrouter. The multicast path is terminated immediately.
IGMP entries
An IGMP entry is allocated for each unique join request, based on the VLAN and IGMP group
address. If multiple ports join the same IGMP group using the same VLAN, only a single
IGMP entry is used.
FastLeave
In a normal IGMP operation, when the switch receives an IGMPv2 leave message, it sends a
Group-Specific Query to determine if any devices in the same group (and on the same port)
are interested in the specified multicast group traffic. The switch removes the affiliated port
from that particular group, if the following conditions apply:
򐂰 If the switch does not receive an IGMP Membership Report within the
query-response-interval.
򐂰 If no multicast routers are learned on that port.
With FastLeave enabled on the VLAN, a port can be removed immediately from the port list of
the group entry when the IGMP Leave message is received, unless a multicast router was
learned on the port.
IGMP v3 snooping
IGMPv3 includes new membership report messages to extend IGMP functionality. The switch
provides snooping capability for all types of IGMP version 3 (IGMPv3) Membership Reports.
IGMPv3 supports Source-Specific Multicast (SSM). SSM identifies session traffic by both
source and group addresses.
The IGMPv3 implementation keeps records on the multicast hosts present in the network. If a
host is already registered, when it receives a new report from same host, the switch makes
the correct transition to new (port-host-group) registration based on the IGMPv3 RFC. The
registrations of other hosts for the same group on the same port are not changed.