To create a flex-algo instance in a network, it is mandatory to configure at least one router to create and advertise the flex-algo definition. This is because the flex-algo definition must be known and distributed throughout the network to enable the desired routing behavior. Other configurations such as defining SRLGs, specifying LSP path computation methods, or configuring LSPs between PE routers might be necessary steps depending on network requirements, but the foundational step is to have at least one router handling the flex-algo definition advertisement.