| 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.
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