Abrer_Wiki

Generic Routing Encapsulation 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.

Assuming all destinations are routable – creating a basic GRE Tunnel over the following topology:

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.

R1–

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

• 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–

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

• At this point of R3, Tu100 is UP/UP and 172.16.0.0/30 is directly connected via Tu100 in the table. Pings should succeed between points of the tunnel. Ping and trace outputs below from R1 to R3's Tu100 172.16.0.2/30

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