Clustering is still an ongoing research area, and the literature in this area is rich. For example, McDonald and Znati  introduce a clustering routing technique for ad hoc wireless LANs, where the network is partitioned into clusters of nodes mutually reachable with a certain specified probability for a certain time. Simulation results support the inherent adaptability and stability of the protocol. Hierarchical clustering and routing techniques have been and continue to be under investigation. ,  A strict hierarchical clustering and routing technique, which was designed for multimedia support in large mobile wireless networks, is presented in Ramanathan and Steenstrup.  Another hierarchical clustering and routing technique, NTDR,  was designed for the tactical environment, where a backbone network exists between cluster heads.
McDonald, B. and Znati, T., A mobility based framework for adaptive clustering in wireless ad hoc networks, IEEE J. Selected Areas Commun., 17 (8), 1–20, 1999.
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Lee, W.C., Topology aggregation in hierarchical routing in ATM networks, ACM Sigcomm '95, Corp. Commun. Rev., 25 (2), 82–92, 1995.
Ramanathan, R. and Steenstrup, M., Hierarchically-organized multihop mobile wireless networks for quality-of-service support, ACM/Blatzer Mobile Networks Appl. J., 3 (1), 101–119, 1988.
Zavgren, J., NTDR mobility management protocols and procedures, Proc. IEEE Military Communications Conference (MILCOM) '97, November 1997.