ECS4660-28F_Management Guide-R03

C HAPTER 15 | Multicast Filtering
IGMP Protocol
– 611 –
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.
Figure 339: IGMP Protocol
LAYER 2 IGMP (SNOOPING AND QUERY FOR IPV4)
IGMP Snooping and Query – If multicast routing is not supported on other
switches in your network, you can use IGMP Snooping and IGMP Query
(page 613) 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.
Network core
(multicast routing)
Edge switches
(snooping and query)
Switch to end nodes
(snooping on IGMP clients)