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Chapter 9

Multicast

F I G U R E 9 . 7

Multicast addressing overlap

The impact that this may have can be significant. This overlap creates a

window for multiple multicast groups' data to be forwarded to and pro-
cessed by machines that didn't intentionally subscribe to the multiple groups.
To give another example, a machine that subscribes to a multicast group
224.2.127.254 would be given a MAC address of 01-00-5e-02-7f-fe. This
host will also process packets that come from multicast group
225.2.127.254 because the layer 2 MAC address is identical.

The problem this creates is that the end host must now process packets

from both multicast groups even though it is only interested in data from
224.2.127.254. This causes unwanted overhead and processor interrupts on
the host machine.

Managing Multicast in an Internetwork

s a user on the network, you can understand that SPAM is not some-

thing that is managed by a systems administrator, whereas valid mailing lists
require maintenance to keep a current list of valid subscribers. The same can
be said of multicast. Reverting a little to the differences between broadcast
and multicast communication, one of the major differences that we discussed

1 1 1 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1

0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 1 1 1 1 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1

01-00-5e-00-01-01

225.128.1.1

1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1

224.0.1.1

Final MAC

multicast address

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