Asante IntraCore 6524, User Manual

Page: 117 / 150

Multicast Traffic Management
Page 4-37
Multicast Traffic Management
Multicast traffic is a means to transmit a multimedia stream from the internet (a
video conference, for example) without requiring a TCP connection from every
remote host that wants to receive the stream. The stream is sent to the multicast
address, and from there it’s propagated to all interested parties on the internet.
Traditional IP communication allows a host to send packets to a single host
(unicast transmission) or to all hosts (broadcast transmission). IP multicast
provides a third scheme, allowing a host to send packets to a subset of all hosts
(group transmission).
Multicast Addresses
Multicasts are sent to special IP addresses in the range from 224.0.0.0 through
239.0.0.0. These are also called “Class D” addresses. The IP multicast address
always begins with the four bits 1110 (which identifies the address as a
multicast). The remaining 28 bits of the multicast address specify the individual
multicast group.
When an end station wants to join in an IP multicast group, it binds the
multicast address of that group to its network interface. When a node is using
an IP multicast address it also uses an Ethernet multicast address. Ethernet IP
multicast addresses begin 01:00:5e. The remaining 24 bits are the lowest 24 bits
of the IP multicast address. There is not a 1-to-1 mapping of IP multicast
addresses to Ethernet multicast addresses.
When configuring a VLAN for multicast traffic, you specify the Ethernet
address for the multicast group. (See “Multicast Forwarding Database
Configuration” on page 4-40.)
IGMP
Communication on a LAN between end stations and routers is managed by the
Internet Group Management Protocol (IGMP). For complete information
about IGMP, see RFC 1112, “Host Extensions...” and RFC 2236, “Internet
Group Management Protocol, Version 2” <ftp://ftp.isi.edu/in-notes/
rfc2236.txt>.
A router that supports multicast and IGMP sends periodic messages called
“queries” on its LAN interfaces. These queries inquire if any end stations want
to join a multicast group. End stations signal their desire to join the multicast
group by responding with an IGMP “report.” By using a multicast routing
protocol, such as Protocol-Independent Multicast (PIM), routers maintain
forwarding tables that they use to forward multicast datagrams.