Which transport mechanism is required for Layer 3 VPNs? 09/18/2018 – by Mod_GuideK 0 Which transport mechanism is required for Layer 3 VPNs? A. GRE B. VXLAN C. IPsec D. MPLS SHOW ANSWERSCorrect Answer: D Explanation/Reference: Download Printable PDF. VALID exam to help you PASS.