D-Link NetDefendOS User Manual page 322

Chapter 4: Routing
If both tunnels must be, for example, IPsec connects, it is possible to wrap IPsec in a GRE
tunnel (in other words, the IPsec tunnel is carried by a GRE tunnel). GRE is a simple tunneling
protocol without encryption and therefore involves a minimum of extra overhead. See
Section 3.4.7, "GRE Tunnels" for more about this topic.
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