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The general form of a token ring functional address is
C0:00:00:04:xx:xx. Functional addresses are used for token ring
functions, such as Ring Error Monitor. The last two bytes usually
have only one bit set to 1 and a bit in the third byte is used to
determine if this address is a functional address. The third byte of
an ethernet multicast address is 5E, which, if used in a token ring
to multicast IP address mapping, would trick the token ring hosts
into accepting that the multicast address is functional. This is the
reason that the same mapping method used for ethernet cannot be used
for token ring. Mapping all IP multicast addresses to the same token
ring functional address means that token ring end stations cannot
determine if the multicast traffic is destined for them until the
packet is examined at layer three. If multicast traffic is present
on the token ring, then every host must examine the packet at layer
three (in software), instead of at layer two (by the network
interface card). This can put a strain on end stations that are not
listening for packets of that particular multicast group. |
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