The bridge sets bits to indicate that it has been copied, which prevents frames from circulating endlessly in the network. The majority of Token Ring networks use source routing, but it is not appropriate in some cases. For example, if you need to connect Token Ring and Ethernet networks, bridging won't be sufficient. You can also interconnect multiple Token Ring networks with routers. Whereas bridges operate at the Data-link layer, routers operate at the Network layer, one level higher.
Routers deliver a higher level of functionality than bridges, but they incur greater overhead.
Many also include bridging functionality for protocols that can't be routed. Companies with Token Ring LANs also look to routing to provide the same level of security, flow control, and path control that Ethernet LANs and routers provide. In a routed network, routers communicate with each other, learn where the end-station devices reside, and manage the traffic flow. They require a specific protocol to do so. Routers can detect congestion on a particular link and send network traffic over a less congested link.