Alcatel Carrier Internetworking Solutions Switch/Router ユーザーズマニュアル

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Switching traffic between like media requires no changes to the frame, whereas switching 
traffic between unlike media requires some level of change to the frame. To fully explain the 
various changes possible we need to define the portion of the frame where changes could 
occur. 

Media Specific fields and MAC address fields are different for Token Ring, FDDI, and Ether-
net. For Token Ring and FDDI, the switch generates MAC addresses in non-canonical format, 
where Ethernet generates MAC addresses in canonical format. The Omni Switch/Router will 
perform media translations which means the media specific, source MAC and destination MAC 
will be changed for each frame which changes media.

The source routing field is optional, and use of this field is driven by endstations who wish to 
communicate using source routing. The Omni Switch/Router participates in source routing on 
FDDI and Token Ring interfaces when it is configured as a Source Route Bridge. The Omni 
Switch/Router will also forward source route frames transparently while performing standard 
switching of frames on Token Ring and FDDI interfaces as well as when using the virtual ring 
feature.

The encapsulation type field can be a number of different encapsulations, which really 
includes the Media Specific fields, source MAC address, and destination MAC address. The 
choices are Ethernet II, IEEE 802.2 (LLC), SNAP, and Novell 802.3 or FDDI proprietary 
formats. There are configuration options for Ethernet, FDDI, and Token Ring interfaces. The 
encapsulation type field may or may not be changed. This decision is made based on the 
incoming encapsulation type, the user configuration, and the topology that frame is traveling. 

The data field is the remainder of the frame which is application dependent. This data field is 
not changed for switched traffic. Each frame is followed by a CRC.

Below are some examples when translation can occur.

Translations are not performed for switched traffic between similar LANs within one Omni 
Switch/Router. For example in the diagram below, if Station A on an Ethernet segment wants 
to talk to Station B on another Ethernet segment, the switched frames are not changed. 

This is true for any two media where the originating media and the destination media are of 
the same type (i.e. Ethernet, FDDI, Token Ring).