The MAC Header
Page 18-4
The MAC Header
The format and values defined for the MAC header are covered in the media standards but
even here a variety of choices which are dictated by the upper layer protocol can be found.
Canonical versus Non-Canonical
The first requirement of the switch transformation is the bit ordering of the address fields. For
Token Ring and FDDI, this is the so called non-canonical ordering or most significant bit
first. For Ethernet, this is canonical or least significant bit first. Thus, when a frame is moved
between these media, the addresses must be bit-swapped.
Abbreviated Addresses
The FDDI and 802.5 Token Ring media allow for the use of small 16 bit addresses or full 48
bit addresses. The Omni Switch/Router only supports 48 bit MAC address LANs thus abbrevi-
ated address based protocols cannot be supported.
Functional Addresses and Multicasts
The 802.5 media also have different rules for the formation of multicast addresses or group
addresses. In Ethernet a single bit defines the address as a multicast. In 802.5 a single bit also
indicates a multicast but the remaining bits are structured into so called Functional Address
groups with pre-assigned meanings and functions.
The Omni Switch/Router does not map MCASTs and Functional Addresses; thus protocols
dependent on these features may not be switchable any to any.
MAC Header RIF Encapsulation Network Header Data
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