2.1 Legacy Media Types
2.1.6 Multicast frames
Multicast frames differ from broadcast frames in a subtle way. Multicast frames address a group of devices with a common interest. The source sends only one copy of the frame on the network, even though it intends for several stations to receive it. When a station receives a multicast frame, it compares the multicast address with its own address. Unless the card is preconfigured to accept multicast frames, the multicast is discarded on the interface and does not consume CPU cycles. (This behaves just like a unicast frame.)

For example, Cisco devices running the Cisco Discovery Protocol (CDP) make periodic announcements to other locally attached Cisco devices. The information contained in the announcement is interesting only to other Cisco devices (and the network administrator). To make the announcement, the Cisco source could send a unicast to each Cisco device. That however, means multiple transmissions on the segment, which consume network bandwidth with redundant information. Furthermore, the source might not know about all the local Cisco devices and could, therefore, choose to send one broadcast frame. All Cisco devices would receive the frame. Unfortunately, so would all third-party devices. The last alternative is a multicast address. Cisco has a special multicast address reserved, 01-00-0C-CC-CC-CC, which enables Cisco devices to transmit to all other Cisco devices on the segment. All third-party devices ignore this multicast message.

Open Shortest Path First (OSPF), an IP routing protocol, sends out routing updates via a specially reserved multicast address. The reserved multicast OSPF IP addresses 224.0.0.5 and 224.0.0.6 translate to MAC multicast addresses of 01-00-5E-00-00-05 and 01-00-5E-00-00-06. Only router interfaces configured to receive OSPF announcements will process these packets. All other devices filter the frame.