Filter
Top Products
Advanced Management
Page 4-44
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 propegated 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 a 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. (However, 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 ehternet
address for the multicast group. (See “Multicast Forwarding Database
Configuration”.)
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