Ethernet sub-rings offer a dual redundant way to interconnect rings. The 7210 SAS supports sub-rings connected to major rings, and a sub-ring connected to a VPLS (LDP based) for access ring support in VPLS networks. Figure: Major ring and sub-ring scenario shows a major ring and sub-ring scenario, and Figure: 0 to 4 G.8032 sub-ring shows a G.8032 sub-ring. In this scenario, any link can fail in either ring (ERP1 or ERP2) and each ring is protected. Also, the sub-ring (ERP2) relies on the major ring (ERP1) as part of its protection for the traffic from C and D. The nodes C and D are configured as interconnection nodes.
Sub-rings and major rings run similar state machines for the ring logic; however, there are some differences. When sub-rings protect a link, the flush messages are propagated to the major ring. (A special configuration allows control of this option on the 7210 SAS.) When major rings change topology, the flush is propagated around the major ring and does not continue to any sub-rings. The reason for this is that major rings are completely connected but sub-rings are dependent on another ring or network for full connectivity. The topology changes need to be propagated to the other ring or network usually. Sub-rings offer the same capabilities as major rings in terms of control and data so that all link resources may be used.