Alcatel Carrier Internetworking Solutions Switch/Router 用户手册

Page 18-4

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.

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.

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.

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.