Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 113
Multicast Support

A key improvement for the RIP protocol is the ability to use a multicast address for its packets and for its datagram IP header. The multicast address for RIPv2 is 224.0.0.9 with a MAC address of 01-00-5E-00-00-09. Of course, this must be mapped to an Ethernet multicast address (for more information on this, please refer to Part Six, BOOTP, DHCP, RSVP, and SNMP in this book, or RFC 1700).

If you read the section on multicasting, you know that the benefits of multicast are great. RIPv1 uses a broadcast address that not only interrupts the NIC but the IP service layer as well, even if the packet is not destined for that host. Why interrupt the host when the packet/datagram is destined for some other host? All broadcast packets must be received and processed. Not a problem when RIPv1 was introduced to the IP community (there were not many hosts to contend with).

Multicast allows only those hosts that have specified their NICs to receive and process multicast packets. All other multicast packets will be ignored.

Even though multicast is used, it is not IGMP (Internet Group Management Protocol) and is not to be used for the address in a local multicast address. This means the packet will never leave the network it was transmitted on (i.e., it will not be forwarded by routers).