Secured and seamless mobility across heterogeneous access networks needs optimization at all layers. Authentication and security association at the link layer is one of the major components of delay during handoff. We propose a network- layer assisted proactive handoff scheme that helps to optimize the handoff process involving link-layer security across multiple subnets. We demonstrate this proactive scheme and analyze the results for IEEE 802.11-based networks for both roaming and non-roaming scenarios. We then compare these results with the pre-authentication techniques offered by IEEE 802.11i.