3Com Switch 8800 Advanced Software V5 Configuration Guide

496 CHAPTER 37: MULTICAST OVERVIEW
IP multicast falls in the scope of end-to-end service. The multicast architecture
involves the following four parts:
1 Addressing mechanism: Information is sent from a multicast source to a group of
receivers through a multicast address.
2 Host registration: Receiver hosts are allowed to join and leave multicast groups
dynamically. This mechanism is the basis for group membership management.
3 Multicast routing: A multicast distribution tree (namely a forwarding path tree for
multicast data on the network) is constructed for delivering multicast data from a
multicast source to receivers.
4 Multicast applications: A software system that supports multicast applications,
such as video conferencing, must be installed on multicast sources and receiver
hosts, and the TCP/IP stack must support reception and transmission of multicast
data.
Multicast Addresses To allow communication between multicast sources and multicast group
members, network-layer multicast addresses, namely, multicast IP addresses must
be provided. In addition, a technique must be available to map multicast IP
addresses to link-layer multicast MAC addresses.
IPv4 multicast addresses
Internet Assigned Numbers Authority (IANA) assigned the Class D address space
(224.0.0.0 to 239.255.255.255) for IPv4 multicast, as shown in
Table 25.
n
Like the 10.0.0.0/8 block that IANA has reserved for IP unicast, 239.0.0.0/8 is an IP
multicast address block reserved by IANA. These addresses are administratively
scoped addresses. The use of the administratively scoped multicast addresses
allows flexible definition of the ranges of multicast domains to isolate addresses
between different multicast domains, so that the same multicast address can be
used in different multicast domains without causing collisions.
The membership of a group is dynamic. Hosts can join or leave multicast groups at
any time.
A multicast group is identified by a multicast address. There are two types of
multicast addresses:
Tabl e 25 Class D IP address blocks and description
Address block Description
224.0.0.0 to 224.0.0.255 Reserved permanent group addresses. The IP address
224.0.0.0 is reserved, and other IP addresses can be
used by routing protocols and for topology
searching, protocol maintenance, and so on.
224.0.1.0 to 231.255.255.255
233.0.0.0 to 238.255.255.255
ASM/SFM multicast addresses available for users (IP
addresses for temporary groups). They are valid
across the internet.
232.0.0.0 to 232.255.255.255 SSM multicast addresses available for users (IP
addresses of temporary groups). They are valid
across the internet.
239.0.0.0 to 239.255.255.255 User-available administratively scoped multicast
addresses for ASM/SFM.