GRE stands for Generic Routing Encapsulation (GRE).It is a protocol that provides service to encapsulate packets in order so that they can be routed over IP network. In nutshell, GRE provides a private point-to-point connection similar to a virtual private network (VPN).
GRE encapsulates a payload -- which is, an inner packet which is required to be delivered to a destination network inside of an IP packet. Payloads are sent through GRE tunnels, by use of routing encapsulated packets between enpdoints. Other IP routers do not open the payload, they open the outer IP packet and forward it to GRE tunnel endpoint. When it reaches tunnel endpoint,removal of GRE encapsulation takes place and the payload is forwarded to destination.
Some advantages of GRE tunnels are mentioned below:
GRE tunnels encase multiple protocols over a single-protocol backbone.
GRE tunnels enable workarounds for networks which have limited hops.
GRE tunnels helps in connecting discontinuous sub-networks.
GRE tunnels help in implementing VPNs across wide area networks (WANs).
When Cisco Express Forwarding (CEF) or fast switching is enabled on the interface and running traffic over an encrypted Generic Routing Encapsulation (GRE) tunnel on a 2811 router, the show interface summary command displays RXBS (receive bit/sec) or Input bits/sec rate counter for the physical interface as zero.
All other counters (such as input/output packet counter, output bits/sec rate, output packets/sec rate, input bits/sec rate) are diplayed fine. Tunnel interface counters also indicate the valid values. It happens with both software-based and hardware-based encryption, but only on 2811. The problem does not occur on 12.3(8)T5 images. It starts on the 12.3(11)T3 image and still exists on the 12.3(12.12)T4 image. The problem does not occur with an unencrypted GRE tunnel.
This issue is documented in Cisco bug ID CSCeh25585.