Filter
Top Products
M
ULTICAST
F
ILTERING
3-191
continue to receive the multicast service. This procedure is called multicast
filtering.
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 and report messages and multicast routing probe
messages to register end-stations as multicast group members, but also
supports the DVMRP and PIM-DM multicast routing protocols required
to forward multicast traffic to other subnets (page 3-323 and 3-334).
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” 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 DVMRP or
PIM, to support IP multicasting across the Internet.