User's Manual

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3.6.3 GRE Tunnels
Generic Route Encapsulation (GRE) defines the encapsulation of any other network layer protocol
on a network layer protocol. GRE could be used as the L3TP of VPN to provide a transparent
transmission channel for VPN data. In simple terms, GRE is a tunneling technology which provides
a channel through which encapsulated data message could be transmitted and encapsulation and
decapsulation could be realized at both ends. GRE tunnel application networking shown as the
following figure:
X Network X Network
GRE Tunnel
Along with the extensive application of IPv4, to have messages from some network layer protocol
transmitted on IPv4 network, those messages could by encapsulated by GRE to solve the
transmission problems between different networks.
In following circumstances GRE tunnel transmission is applied:
 GRE tunnel could transmit multicast data packets as if it were a true network interface.
Single use of IPSec cannot achieve the encryption of multicast.
 A certain protocol adopted cannot be routed.
 A network of different IP address shall be required to connect other two similar networks.
GRE application example: combined with IPSec to protect multicast data
GRE can encapsulate and transmit multicast data in GRE tunnel, but IPSec, currently, could only
carry out encryption protection against unicast data. In case of multicast data requiring to be
transmitted in IPSec tunnel, a GRE tunnel could be established first for GRE encapsulation of
multicast data and then IPSec encryption of encapsulated message so as to achieve the
encryption transmission of multicast data in IPSec tunnel. As shown below: