Answers: GRE Tunnel 3

By Chris July 22, 2016 13:10

You know the drill. Do the lab, come here to check your answer. This one is on configuring a point-to-point GRE tunnel, using a hostname for the tunnel destination, and with OSPF interface thrown in for some quick review.


Figure 1: Two Enterprise Routers at Acme Corp, with One Link Acting as Entire Internet

Example 1: Edge1 Config

Example 2: Edge2 Config



Many enterprises use GRE tunnels, either directly as configured in this lab, or multipoint GRE tunnels as implemented with DMVPN. Getting a good handle on the basic GRE encapsulation and configuration is a good beginning step for a CCNA to learn before moving into the details of configuring IPsec encryption to be used over that tunnel.

For this lab you are tasked with configuring a manual point-to-point GRE tunnel between Edge routers, and then configuring OSPF over this newly configured tunnel. This lab differs from the earlier labs because it utilizes tunnel source IP addresses (instead of interfaces), and tunnel destination hostnames (instead of addresses).

First, think about the big picture. The routers will use these IP addresses as the tunnel source and destination, per the requirements in the lab plus the network diagram:

Edge1: Source is, destination is

Edge2: Source is, destination is

The lab instructions ask you to configure the source IP address directly (that is, do not reference the interface in the tunnel source command). So, looking at Examples 1 and 2, the two tunnel source commands simply refer to the IP addresses just mentioned: (Edge1) and (Edge2).

Next, the lab asked you to use a locally-defined hostname to refer to the tunnel destination. The Examples use obvious names, for instance, using the ip host edge2 command on Edge1 to define a hostname for Edge2’s address. Similarly, Edge2’s configuration uses the ip host edge1 command. Once configured, the tunnel destination commands can just reference those hostnames, with the tunnel destination edge2 command on router Edge1 and the tunnel destination edge1 command on router Edge2.

At this point, the GRE tunnel itself has been configured, and the next step is to enable IPv4 and then OSPF. Per the figure, Edge1 uses address, with Edge2 using, both with mask /30. Then both routers’ tunnel interfaces need the ip ospf 10 area 0 command, which enables OSPF for OSPF process 10, and places the tunnel interface into area 0. Note that OSPF process 10 had already been configured on both Edge1 and Edge2 per the initial configuration; that initial configuration is how you would know to use a “10” in the command as the OSPF process ID.

GRE Tunnel 3
By Chris July 22, 2016 13:10
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  1. stefano August 29, 07:12

    I think that the address in the example 1 and are not correct.
    you should write ip address and ip address instead of ip address and

    Reply to this comment
  2. Efrain January 19, 08:16

    Buen laboratorio.

    Gracias por compartirlo.

    Reply to this comment
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