Ethernet Sub-Rings offer a dual redundant way to interconnect rings. The router supports Sub-Rings connected to major rings and a sub-ring connected to a VPLS (LDP based) for access rings support in VPLS networks. Figure: 0-4 G.8032 sub-ring illustrates a Major Ring and Sub-Ring scenario. In this scenario, any link can fail in either ring (ERP1 or ERP2) and each ring is protected. Furthermore, 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 inter connection 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 router.) 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 resource may be used.