Chapter 14
| Multicast Filtering
Layer 2 IGMP (Snooping and Query for IPv4)
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Note:
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:
â–
Upon the expiration of a periodic (randomized) timer.
â–
As a part of a router's start up procedure.
â–
During the restart of a multicast forwarding interface.
â–
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:
â–
Multicast forwarding is disabled on an interface.
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An interface is administratively disabled.
â–
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.
Note:
MRD messages are flooded to all ports in a VLAN where IGMP snooping or
routing has been enabled. To ensure that older switches which do not support MRD
can also learn the multicast router port, the switch floods IGMP general query
packets, which do not have a null source address (0.0.0.0), to all ports in the
attached VLAN. IGMP packets with a null source address are only flooded to all
ports in the VLAN if the system is operating in multicast flooding mode, such as
when a new VLAN or new router port is being established, or an spanning tree
topology change has occurred. Otherwise, this kind of packet is only forwarded to
known multicast routing ports.
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