Internet Group Management Protocol; Multicast Routing Protocol - Huawei Quidway S3500 Series Operation Manual

Operation Manual - Multicast
Quidway S3500 Series Ethernet Switches
PIM-SM, PIM-DM and MSDP. While the unicast routing protocol BGP can also be
expanded to support transmitting multicast routing protocol information between
domains.

1.3.1 Internet Group Management Protocol

Internet Group Management Protocol is the only protocol that hosts can use. It defines
the membership establishment and maintenance mechanism between hosts and
routers and is the basis of the entire IP multicast. Hosts report the group membership to
a router through IGMP and inform the router of the conditions of other members in the
group through the directly connected host. If a user on the network joins a multicast
group through IGMP declaration, the multicast router on the network will transmit the
information sent to the multicast group through the multicast routing protocol. Finally,
the network will be added to the multicast tree as a branch. When the host, as a
member of a multicast group, begins receiving the information, the router will query the
group periodically to check whether members in the group are involved. As long as one
host is involved, the router will continue to receive data. When all users on the network
quit the multicast group, the related branches are removed from the multicast tree.

1.3.2 Multicast Routing Protocol

A multicast group address is a virtual address. Unicast allows packets to be routed from
the data source to the specified destination address, which is impossible for multicast.
The multicast application sends the packets to a group of receivers (with multicast
addresses) who want to receive the data but not only to one receiver (with unicast
address).
The multicast routing creates a loop-free data transmission path from one data source
to multiple receivers. The task of the multicast routing protocol is to build up the
distribution tree architecture. A multicast router can use multiple methods to build up a
path for data transmission, i.e., the distribution tree.
PIM-DM (Protocol-Independent Multicast Dense Mode, PIM-DM)
PIM dense mode is suitable for small networks. It assumes that each subnet in the
network contains at least one receiver who is interested in the multicast source.
Therefore, multicast packets are flooded to all points of the network. Subsequent
resources related (such as bandwidth and CPU of routers) will be consumed. In order
to decrease the consumption of these precious network resources, branches that do
not have members send Prune messages toward the source to prune off the
unwanted/unnecessary traffic. To enable the receivers in the pruned branches who
have multicast data forwarding requirement to receive multicast data streams, the
pruned branches can be restored to forwarding state periodically. To reduce the latency
time during which the pruned branches wait for being restored, PIM dense mode uses
the prune mechanism to actively restore the forwarding of multicast packets. The
periodical flood and prune are characteristics of PIM dense mode. Generally, the
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Chapter 1 IP Multicast Overview