The third layer of the tunnel is not a new technology. The GRE is defined in RFC1701 for a long time already. CISCO ios version since 9.21 to the support of the technology. Ipsec Encryption is a new tunnel to support the definition of the IETF standards. CISCO ios version 11.3T support since the features. CISCO ios version 12.0T in mobile IP. GRE tunnel is based on IP technology, which can be used in IP-based multi-protocol backbone network transmission of data traffic such as IPX, AppleTalk and so on. Meanwhile, GRE can also be used to transmit Internet radio through the tunnel on the network and multicast information such as routing update information. Note that, in the need to use the GRE before the end of the VPN device as a physical interface on the configuration, then you can use, such as IPSec and other security measures to protect the tunnel.
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