Web Management Guide

Chapter 13 | Multicast Filtering
IGMP Protocol
– 434 –
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).
This switch not only supports IP multicast filtering by passively monitoring IGMP
query, report messages and multicast routing probe messages to register end-
stations as multicast group members, but also supports the Protocol Independent
Multicasting (PIM) routing protocol required to forward multicast traffic to other
subnets (page 607 and 623).
IGMP Protocol
The Internet Group Management Protocol (IGMP) runs between hosts and their
immediately adjacent multicast router/switch. IGMP is a multicast host registration
protocol that allows any host to inform its local router that it wants to receive
transmissions addressed to a specific multicast group. 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” (at Layer 3) and assumes the
role of querying the LAN for group members. It then propagates the service
requests on to any adjacent multicast switch/router to ensure that it will continue
to receive the multicast service. Based on the group membership information
learned from IGMP, a router/switch can determine which (if any) multicast traffic
needs to be forwarded to each of its ports. At Layer 3, multicast routers use this
information, along with a multicast routing protocol such as Protocol Independent
Multicasting (PIM), to support IP multicasting across the Internet. Note that IGMP
neither alters nor routes IP multicast packets. A multicast routing protocol must be
used to deliver IP multicast packets across different subnetworks. Therefore, when
PIM routing is enabled for a subnet on the switch, IGMP is automatically enabled.