RFC 1469 (rfc1469) - Page 2 of 4

IP Multicast over Token-Ring Local Area Networks

RFC 1469           IP Multicast over Token-Ring LANs           June 1993


   It would be most desirable if Token-Ring could use the same mapping
   as ethernet and fddi for IP multicast to hardware multicast
   addressing.  However, current implementations of Token-Ring
   controller chips cannot support this. To see why, we must first
   examine the Destination MAC address format.

Destination Address Format

   The destination MAC address consists of six octets.  In the following
   diagram of a MAC address, the order of transmission of the octets is
   from top to bottom (octet 0 to octet 5), and the order of
   transmission of the bits within each octet is from right to left (bit
   0 to bit 7).  This is the so-called "canonical" bit order for IEEE
   802.2 addresses.  Addresses supplied to or received from token ring
   interfaces are usually laid out in memory with the bits of each octet
   in the opposite order from that illustrated, i.e., with bit 0 in the
   high-order (leftmost) position within the octet.

            7   6   5   4   3   2   1   0

          ---------------------------------
          |   |   |   |   |   |   |U/L|I/G|       octet 0
          ---------------------------------
          |   |   |   |   |   |   |   |   |       octet 1
          ---------------------------------
          |   |   |   |   |   |   |   |FAI|       octet 2
          ---------------------------------
          |   |   |   |   |   |   |   |   |       octet 3
          ---------------------------------
          |   |   |   |   |   |   |   |   |       octet 4
          ---------------------------------
          |   |   |   |   |   |   |   |   |       octet 5
          ---------------------------------

   The low order bit of the high order octet is called the I/G bit. It
   signifies whether the address is an individual address (0) or a group
   address (1). This is comparable to the multicast bit in the DIX
   Ethernet addressing format.

   Bit position 1 of the high order octet, called the U/L bit, specifies
   whether the address is universally administered (0) or locally
   administered (1). Universally administered addresses are those
   specified by a standards organization such as the IEEE.

   If the I/G bit is set to 1 and the U/L bit is 0, the address must be
   a universally administered group address. If the I/G bit is 1 and the
   U/L bit is a 1, the address may be either a local administered group
   address or a functional address. This distinction is determined by



Pusateri