Combining GRE with IPSec to Protect Multicast Data
GRE can encapsulate multicast data and the data in a GRE tunnel. As show in Figure 3-7, for multicast data that needs to be transmitted over an IPSec tunnel, you can set up a GRE tunnel, encapsulate multicast data with GRE, encrypt the encapsulated data with IPSec, and then transmit the data over the IPSec tunnel.