Encrypted vs Nonencrypted VPNs
In encrypted VPNs, encryption mechanisms are used to secure the traffic across the public network.
Example: IPsec VPNs
In nonencrypted VPNs, either data security is not ensured at all, or is ensured by other means (including encryption at higher layers).
MPLS VPNs (Multiprotocol Label Switching)
GRE-based VPNs (ch. 11)
Uses higher layer encryption for confidentiality