Recommendation ITU-T G.9905 specifies CMSR (Centralized Metric based Source Routing), a proactive, Layer 2 multi-hop routing protocol. CMSR is a proactive routing protocol which can find and maintain reliable routes considering the link quality of both directions. The routing control packet overhead of CMSR is quite low compared to existing proactive routing protocol such as OLSR, so that it can be applied for large scale network even on narrow band PLC.