There are essentially two kinds of bridges. The first type is a Source Route Bridge, which allows end devices to request a particular path through the network, using a Routing Information Field (RIF) in the packet. In the default case, this type of bridge cannot forward any packet without a RIF. The second type is a Transparent Bridge, which hides all of that network detail from end devices. Transparent bridges have no concept of a RIF. Source Route Bridging is commonly used with Token Ring networks, while Transparent Bridging is popular with Ethernets, where it is used by Ethernet switches.
So bridging between Ethernet and Token Ring networks requires a special hybrid of these two that is able to translate between not only the media types, but also the bridging types. The Remote Source Route Bridging (RSRB) and Source Route Transparent (SRT) bridging protocols were invented to solve this problem, particularly over WANs.
Data Link Switching (DLSw) and DLSw+, which is Cisco’s enhanced version of DLSw, also solve these problems and comply with the same bridging standards. These protocols are capable of connecting Token Rings to Ethernets, Synchronous Data Link Control (SDLC) serial connections, and even X.25 networks. So there is really very little reason to worry about the older bridging protocols and methods anymore. If you are considering building a new network involving the System Network Architecture (SNA) protocol, there is no particular reason to bother with either ...