• Now that you understand why GRE is used, we can build the tunnel.

    In this lab, R1 and R2 are connected through two ISP routers across the Internet.
    The Internet acts as the underlay network, while the GRE tunnel will create the overlay connection between the two sites.

    GRE tunnel lab topology with ISP transport network

    Figure 1 - GRE lab topology

    Before creating the GRE tunnel, the underlay network must be working correctly.
    This means the ISP routers must be able to route traffic between the tunnel endpoints.

    First, configure static routes on the ISP routers so they can forward traffic between the tunnel endpoints.
    This ensures the underlay network can reach the GRE tunnel endpoints.

    ISP1 Static Routing

    Static routes configured on ISP routers for GRE lab connectivity

    Figure 2 - ISP1 static routing

    ISP1 must know how to reach the remote WAN link behind ISP2.
    On ISP1, configure the following static route:

    ISP1# conf t
Enter configuration commands, one per line.  End with CNTL/Z.
ISP1(config)# ip route 203.0.113.8 255.255.255.252 203.0.113.6

    This route allows ISP1 to forward traffic toward the remote WAN subnet behind ISP2.

    ISP2 Static Routing

    ISP2 must also know how to reach the remote WAN link behind ISP1.

    static route configuration on ISP2

    Figure 3 - ISP2 static routing

    Configure the following route on ISP2:

    ISP2# conf t
Enter configuration commands, one per line.  End with CNTL/Z.
ISP2(config)# ip route 203.0.113.0 255.255.255.252 203.0.113.5

    At this stage, the underlay network can route traffic between R1 and R2.

    Configure Default Route

    Each enterprise router must also have a default route pointing toward the ISP.

    Default route configuration to reach GRE tunnel destination

    Figure 4 - Default route configuration

    On R1:

    R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.2

    On R2:

    R2(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.9

    These routes ensure that Internet-bound traffic is forwarded to the ISP routers.

    Answer the question below

    What is the next hop of the default route configured on R1?