User's Manual

Configuring GRE, NAT, RIPSO, and BFE Services
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308625-14.00 Rev 00
GRE Concepts and Terminology
Generic Routing Encapsulation (GRE) is a protocol that allows transport of
non-IP traffic through IP-based systems. GRE, which is defined in RFCs 1701 and
1702, encapsulates Internet Protocol (IP) and other layer 3 protocols to enable
data transmission through an IP tunnel. This tunneling mechanism allows:
• Transport of non-IP traffic through intermediate systems that support only IP
• Creation of a virtual private network (VPN) that uses the Internet as a section
of your own private network
• Communication between subnetworks with unregistered or discontiguous
network addresses
A tunnel is a virtual point-to-point connection. It has as its end points the IP
addresses of two router IP interfaces, one serving as the source, the other serving
as the destination. When using GRE, remember that:
• This protocol is slower than native routing because packets require additional
processing.
• IP fragmentation of the packet can occur due to extra bytes introduced by
encapsulation.
• Troubleshooting the physical link when problems occur is difficult.
GRE tunnels support encapsulation of the IP, IPX, and OSI protocols.