Cisco Systems RJ-45-to-AUX Delivery of Multicast Datagrams, Multicast Distribution Tree, 142

Address	Destination
.1	All systems in this subnet
224.0.0.2	All routers in this subnet
.4	Distance Vector Multicast Routing Protocol
.5	Open Shortest Path First (OSPF) routers
.6	Open Shortest Path First (OSPF) designated routers
.7	Routing Information Protocol 2 (RIP2) routers
.8	Internet Gateway Routing Protocol (IGRP) routers
.9	Dynamic Host Configuration Protocol (DHCP)
	server/relay agent
.10	All Protocol−Independent Multicast (PIM) routers

The range from 224.0.1.0 to 224.1.255.255 is assigned by IANA to network protocols and network applications on a permanent basis. To obtain one of these addresses, you must have a really good technical justification.

The range from 239.0.0.0 to 239.255.255.255 contains administratively scoped addresses for use in private networks, similar to the use of 10.x.x.x for IP addresses. The network administrator must ensure that this kind of multicast traffic does not leave the private network.

The range from 224.2.0.0 to 233.255.255.255 is dynamically assigned to applications that require a multicast address. This process is similar to the way Dynamic Host Configuration Protocol (DHCP) assigns IP addresses to hosts. The multicast address is allocated or leased when it’s needed and released to be used by others when it’s not needed. The Session Directory (SDR) program currently is the most widely used method of dynamically allocating IP multicast addresses. The Internet Engineering Task Force (IETF) is considering modifying SDR to enable it to scale well or to define and develop a new form of dynamic multicast address allocation.

Note IANA has started to assign blocks of multicast addresses from this range. To review a current list of multicast addresses, visit www.isi.edu/in−notes/iana/assignments/multicast−addresses.

Delivery of Multicast Datagrams

When the sender and recipients are members of the same subnet, delivery of multicast frames is a simple matter. The source station addresses the IP packet to the multicast group, the Network Interface Card (NIC) maps the IP address to the multicast MAC address, and the frame is sent. Recipients that want to capture the frame notify their IP layer that they want to receive datagrams addressed to the group.

When the sender is on one subnet and the recipients are on different subnets, the routers need to use a multicast routing protocol that permits the construction of multicast delivery trees and supports multicast data packet forwarding.

Multicast Distribution Tree

The multicast model uses one source and many destinations. The packet travels from one router to many routers on different paths. This route forms a multicast distribution tree, with the first router as the root. Multicast distribution tree is a dynamic visualization of how the multicast packet travels the network. The packet travels from one router to the next until it reaches the destination—it follows a line from end to end.

Note In unicast, there is no such tree.

There are two types of distribution trees: source trees and shared trees.

The source tree is the simplest type of tree. The source of the multicast traffic forms the root of the tree and the branches are the paths through the network to the destination. This form is commonly referred to as the

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