User's Manual
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Chapter 71 Multicast Overview
The chapter describes how to cofigure the multicast routing protocol. For the details of the multicast routing
commands, refer to the part “Multicast Routing Commands”.
The traditional IP transmission allows only one host to communicate with a single host (unicast communication)
or to communicate with all hosts (broadcast communication). The multicast technology allows one host to send
message to some hosts. These hosts are called as group members.
The destination address of the message sent to the group member is a D-class address
(224.0.0.0~239.255.255.255). The multicast message is transmitted like UDP. It does not provide reliable
transmission and error control as TCP does.
The sender and the receiver make up of a multicast application. The sender can send the multicast message
without joining in a group. However, the receiver has to join in a group before it receives the message from the
group.
The relationship between group members is dynamic. The host can join in or leave a group at any
time. There is no limitation to the location and number of the group member. If necessary, a host
can be a member of multiple groups. Therefore, the state of the group and the number of group
members varies with the time.
The router can maintain the routing table for forwarding multicast message by executing the multicast routing
protocol such as PIM-DM and PIM-SM. The router learns the state of the group members in the directly-
connected network segment through IGMP. The host can join in a designated IGMP group by sending the
IGMP Report message.
The IP multicast technology is suitable for the one-to-multiple multimedia application.
71.1 Multicast Routing Realization
In the router software of our router, the multicast routing includes the following regulations:
IGMP runs between the router and the host in the LAN, which is used to track the group
member relationship.
OLNK is a static multicast technology, which is used in the simple topology. It realizes the
multicast forwarding and effectively saves CPU and bandwidth.
PIM-DM, PIM-SM and DVMRP is dynamic multicast routing protocols. They run between
routeres and realizes the multicast forwarding by creating the multicast routing table.
The following figure shows the multicast protocols used in the IP multicast applications: