Bridging IP between two different LANs, such as Token Ring and Ethernet, is usually referred to as translational bridging. Any layer two frame traveling from Ethernet to Token Ring and vice versa requires translation because Token Ring and Ethernet frames have different formats, as illustrated in Figure 7-1.
Normal bridging forwards layer two frames from one interface to another unchanged. However, the interfaces must be of the same physical type and the layer two encapsulations must be the same. A non-translating bridge cannot change an Ethernet Type II encapsulated frame into an 802.3 encapsulated Ethernet frame. A translating bridge, on the other hand, can. Figure 7-1 shows examples of three common frame types.