Since network configurations in the Internet are getting complicated, managing networks has become a heavy task. As an attempt to solve this problem, the concept of Intent-Based Networking (IBN) has been proposed. While intent of a network administrator in conventional networks is used to be expressed in concrete description of configurations stored on devices each, intent of an administrator in IBN is expressed abstractly and prescriptively as what he wants to do. The concept of Intent-Based Network Modeling (NEMO) language has been discussed in IETF IB-Nemo BoF, and a draft specification and implementation of it is developed in the NEMO project. NEMO, a transaction based Northbound API, currently supports description of simple proactive policy only such as change of network path at a specified time. We propose a mechanism that enables an administrator to update network configuration automatically in accordance with the change of external environment, based on reactive configurations which express the intent of the administrator. In this paper, we will describe automatic network management methods that have been studied, and then describe our proposed procedure to update configuration reactively in an extended NEMO language. Issues on implementation are also discussed.