RFC 1469 (rfc1469) - Page 3 of 4

IP Multicast over Token-Ring Local Area Networks

RFC 1469           IP Multicast over Token-Ring LANs           June 1993


   the Functional Address Indicator (FAI) bit located in bit position 0
   of octet 2.  If the FAI bit is 0, the address is considered a
   functional address.  And if the FAI bit is 1, this indicates a
   locally administered group address.

   Different functional addresses are made by setting one of the
   remaining 31 bits in the address field. These bits include the 7
   remaining bits in octet 2 as well as the 8 bits in octets 3, 4, and
   5. It is not possible to create more functional addresses by setting
   more than one of these bits at a time.

   Three methods exist for mapping between an IP multicast address and a
   hardware address. These include:

      1.   The all rings broadcast address

      2.   The assigned functional address

      3.   The existing IEEE assigned IP Multicast group addresses

   In order to insure interoperability, all systems supporting IP
   multicasting on each physical ring must agree on the hardware address
   to be used. Therefore, the method used should be configurable on a
   given interface.  Bridges may provide a means to translate between
   different methods for each physical ring that is being bridged.
   Method (3) is recommended but due to hardware limitations of Token-
   Ring controller chips, may not be possible. In this case, Method (2)
   is preferred over Method (1).  For backward compatibility, systems
   that support (2) MUST also support (1). And systems that support (3)
   MUST also support (2) and therefore (1).  In the absence of
   configuration information, the default should be to use the assigned
   functional address (2).

IP Multicast Functional Address

   Because there is a shortage of Token-Ring functional addresses, all
   IP multicast addresses have been mapped to a single Token-Ring
   functional address. In canonical form, this address is 03-00-00-20-
   00-00.  In non-canonical form, it is C0-00-00-04-00-00.  It should be
   noted that since there are only 31 possible functional addresses,
   there may be other protocols that are assigned this functional
   address as well.  Therefore, just because a frame is sent to the
   functional address 03-00-00-20-00-00 does not mean that it is an IP
   multicast frame.







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