Ericsson ECN330, User Manual

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Configuring the ECN330-switch
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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).
The ECN330-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
DVMRP and PIM multicast routing protocols required to forward multicast traffic
to other subnets (page 473 and 484).
6.16.1 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 Distance Vector Multicast Routing Protocol (DVMRP)
or 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 DVMRP or PIM routing is enabled for a subnet on
the ECN330-switch, IGMP also needs to be enabled.