=====GRE=====
**G**eneric **R**outing **E**ncapsulation protocol can be used as logical point-to-point links, encapsulating most any traffic over a tunnel as unicast. The unicast feature being useful over IPSEC as IPSEC only works over unicast traffic.

====Configuration Example====
=== Topology ===
Assuming all destinations are ''**routable**'' -- creating a basic ''**GRE Tunnel**'' over the following topology:
{{:icnd1:gre-topology.png?nolink|}}

Simple Topology--
  * 3 routers / 2 subnets
  * R1 and R3 Loopback0 interfaces
  * We will create the 172.16.0.0 /30 GRE Tunnels over R1 and R3.



=== Tunnel Configuration ===
R1--
<code>
R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#int tunnel 100
R1(config-if)#tunnel source loopback 0
R1(config-if)#tunnel destination 10.35.0.1
*Mar  1 00:01:05.803: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel100, changed state to up
</code>

  * At this point of R1, Tu100 is UP/UP and 172.16.0.0/30 is directly connected via Tu100 in the table. Pings to 172.16.0.2 fail until R3 is configured.

R3--
<code>
R3#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R3(config)#interface tunnel 100
R3(config-if)#ip address 172.16.0.2 255.255.255.252
R3(config-if)#tunnel source loopback 0
R3(config-if)#tunnel destination 10.15.0.1
*Mar  1 00:12:56.219: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel100, changed state to up
</code>

  * Ping and traceroute results below from R1 to R3 via Lo0 and Tu100.

<code>
Gateway of last resort is not set

     172.16.0.0/30 is subnetted, 1 subnets
C       172.16.0.0 is directly connected, Tunnel100
     10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C       10.10.0.0/24 is directly connected, FastEthernet0/0
C       10.15.0.1/32 is directly connected, Loopback0
O       10.20.0.0/24 [110/20] via 10.10.0.2, 00:11:28, FastEthernet0/0
O       10.35.0.1/32 [110/21] via 10.10.0.2, 00:11:28, FastEthernet0/0


R1#ping 172.16.0.2 <-- R3 Tu100 interface
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.0.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/22/32 ms

R1#traceroute 172.16.0.2 <-- R3 Tu100 interface

Type escape sequence to abort.
Tracing the route to 172.16.0.2

  1 172.16.0.2 44 msec 40 msec 40 msec
  
  
R1#traceroute 10.35.0.1 <-- R3 Lo0 interface

Type escape sequence to abort.
Tracing the route to 10.35.0.1

  1 10.10.0.2 40 msec 8 msec 12 msec
  2 10.20.0.2 16 msec 36 msec 24 msec
</code>