In multi-ring and ladder topologies, you can enable the multi-ring feature by means of the sub-ring command.
Interconnected rings consist of one major ring and at least one sub-ring within the same VLAN. A sub-ring is not a complete ring. Nodes within a sub-ring can be configured as a one-arm ring. Each sub-ring must have its own RPL owner and RPL ports as appropriate.
RPL ports and the RPL owner also must be configured in a sub-ring. All ERP features are available in both major rings and sub-rings.
R-APS PDUs flow only in the nodes with same ring ID. The R-APS PDU can be forwarded through the port in sub-ring blocking state.
The parent-ring-id keyword is used to associate the sub-ring with its parent ring. In such a case, use the parent-ring-id configuration to determine the ring to which the sub-ring is connected. The parent ring can be either another sub-ring or a major ring connected to the sub-ring.
Use the no sub-ring command to delete the sub-ring support.
If you have six nodes you can put them in one ring. The latency time for packet transport, however, increases in big topologies even within the same VLAN, so it is better to separate them out.
The following example sets the sub-ring as well as a parent-ring ID of 2.
device# configure terminal device(config)# erp 1 device(config-erp-1)# sub-ring parent-ring-id 2