Cisco Systems, Inc Cisco Systems, Inc Cisco Systems, Inc This document extends [RFC6551] by defining a new DODAG metric called DODAG size, which can be used for DODAG selection in Low-Power and Lossy Networks (LLNs). DODAG size is an important metric for nodes to decide which DODAG to join, or which DODAG to migrate. This document proposes methods to disseminate DODAG size from the Root to all nodes in the DODAG, so that the DODAG size can be advertised to new joining nodes.