This document summarizes several cluster-based routing protocols for wireless sensor networks that aim to improve energy efficiency. It discusses both pre-established protocols like LEACH, EEHC, and HEED, as well as on-demand protocols like Passive Clustering and Energy Level-based Passive Clustering. For each protocol, it describes the key ideas such as random cluster head election, using residual energy to select cluster heads, and rotating cluster head roles to balance energy load among nodes. The document concludes that hierarchical routing protocols can improve energy efficiency but optimal clustering parameters and handling of network dynamics are still challenges.