In this work we point out the issue of the IETF RPL routing protocol's two different downwards routing schemes not being able to interoperate with each other. This problem is less of an issue when low-power and lossy networks (LLNs) are deployed homogeneously but with the industrial kickoff and large scale deployments, the interoperability of heterogeneous, standards-compliant implementations will become a significant issue. To address this, we suggest two major changes to IETF RPL (RFC 6550). First we suggest that all storing mode nodes should hold the capability to understand and attach source routing headers that the non-storing mode nodes require to forward packets. Next, we suggest that RPL's nonstoring mode nodes should send their destination advertisement messages hop-by-hop, rather than the current end-to-end approach. We show, with two different IPv6 implementations in TinyOS and NanoQplus, that our suggestions help achieve high interoperability performance among different implementations for downwards traffic patterns.