Enterprises want two things from a site-to-site VPN: encryption across an untrusted network, and dynamic routing so that sites learn each other's prefixes and fail over automatically. GRE alone gives you routing but no encryption. A classic crypto map gives encryption but does not carry multicast, so routing protocols such as OSPF and EIGRP, which use multicast hellos, do not work across it. Two designs combine the strengths: GRE over IPsec and IPsec virtual tunnel interfaces (VTIs).
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