Sometimes the only reason to connect Token Rings to an Ethernet is to use the backbone bandwidth of the Ethernet. For example, systems on one Token Ring might need to reach an SNA host on another Token Ring, and might have no need to reach any system connected to the Ethernet.
In this case, there is no reason to translate between Ethernet and Token Ring MAC formats because Token Ring and Ethernet systems do not want to communicate with one another.
Several vendors offer proprietary tunneling solutions that suit this situation. Figure 17.11 illustrates how it works. The following is true for tunneling bridges:
The bridges carry each Token Ring frame ...