Spanning tree protocol is used in layer 2 networks for logical loop avoidance because laer 2 PDU doesn’t have any TTL value defined in their header like layer 3 packets does. Convergence and scalability is the biggest challenges for the evolving Metro Ethernet networks. RPR aka Resilient Packet Ring was proposed by IEEE (802.17) with 50ms convergence with new mac 802.17 header enhancements, which was not compatible with existing Ethernet header consequences lot of new efforts required to build a new code.
So a protocol is require which could be fitted in the current Ethernet header with less efforts and provides faster convergence than the legacy protocols. ITU came up with Ethernet Ring Protection Protocol aka ERPS(G.8032) which uses the SONET/SDH APS(Automatic Protection Switching) protocol for protection switching and Ethernet operation and maintenance for detecting of faster failures which is less than 50ms.
In an Ethernet ring, without congestion, with all nodes in the idle state with less than 1’200 km of ring fibre circumference, and fewer than 16 to 255 nodes, the switch completion time (transfer time) for a failure on a ring link shall be less than 50 ms. In fact, practically it is not possible.