LevelOne GTL-2691 - Manual (Page 523)

C HAPTER 15 | Multicast Filtering
Layer 2 IGMP (Snooping and Query)
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and snooping switches from different vendors. In response to this problem,
the Multicast Router Discovery (MRD) protocol has been developed for use
by IGMP snooping and multicast routing devices. MRD is used to discover
which interfaces are attached to multicast routers, allowing IGMP-enabled
devices to determine where to send multicast source and group
membership messages. (MRD is specified in draft-ietf-magma-mrdisc-07.)
Multicast source data and group membership reports must be received by
all multicast routers on a segment. Using the group membership protocol
query messages to discover multicast routers is insufficient due to query
suppression. MRD therefore provides a standardized way to identify
multicast routers without relying on any particular multicast routing
protocol.
N OTE :
The default values recommended in the MRD draft are implemented
in the switch.
Multicast Router Discovery uses the following three message types to
discover multicast routers:
◆ Multicast Router Advertisement – Advertisements are sent by routers to
advertise that IP multicast forwarding is enabled. These messages are
sent unsolicited periodically on all router interfaces on which multicast
forwarding is enabled. They are sent upon the occurrence of these
events:
■
Upon the expiration of a periodic (randomized) timer.
■
As a part of a router's start up procedure.
■
During the restart of a multicast forwarding interface.
■
On receipt of a Solicitation message.
◆ Multicast Router Solicitation – Devices send Solicitation messages in
order to solicit Advertisement messages from multicast routers. These
messages are used to discover multicast routers on a directly attached
link. Solicitation messages are also sent whenever a multicast
forwarding interface is initialized or re-initialized. Upon receiving a
solicitation on an interface with IP multicast forwarding and MRD
enabled, a router will respond with an Advertisement.
◆ Multicast Router Termination – These messages are sent when a router
stops IP multicast routing functions on an interface. Termination
messages are sent by multicast routers when:
■
Multicast forwarding is disabled on an interface.
■
An interface is administratively disabled.
■
The router is gracefully shut down.
Advertisement and Termination messages are sent to the All-Snoopers
multicast address. Solicitation messages are sent to the All-Routers
multicast address.