GRE Tunnel 3

By Chris July 21, 2016 13:05

This latest lab requires you to configure a point-to-point GRE tunnel, but with no encryption config. Unless this is your first Config Lab, you know the drill. If not: take 5-10 minutes now, read the lab, and type your answer or write it on paper. Then look for the answer post (which usually follows within a day or two if you happen to read this the day it was posted). Check here for all ICND2 Config Labs.


Think of the four routers in this lab design as four enterprise routers. All sit in OSPF area 0. However, that one physical link between routers Edge1 and Edge2, now think of that as the entire Internet. (For simplicity in lab, this lab exercise uses that one link as the entire Internet.) So:

  • The link between Edge1 and Edge2 are considered to be using public addresses
  • All other links use private address (in private network

From a GRE tunnel perspective, you will use the public IP addresses on the link between Edge1 and Edge2 as the tunnel source and destination. Then, for the tunnel interface itself, you will place the interfaces into a subnet in private network, you will use OSPF to exchange routes from private network, with all subnets in area 0.

The specific rules for this lab are:

  • As the tunnel source, use the IP address on the interfaces connecting the Edge routers. (That is, do not refer to the interfaces as the tunnel source.)
  • As the tunnel destination, create a local hostname referencing the other Edge router’s Internet-facing IP address (will be interface G0/1 on each router). Then use that hostname as the tunnel destination.
  • Assign the tunnel interfaces private IPv4 addresses from network per the figure.
  • Configure OSPFv2 in area 0 on the tunnel interface using interface configuration (that is, do not use OSPF network commands).
  • Make sure the new OSPF configuration agrees with the existing configuration.
  • Assume all device interfaces shown in the lab are up, working and with correct IP addresses assigned.


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


Initial Configuration

Examples 1, 2, 3 and 4 show the beginning configuration state of Edge1, Edge2, Acme1 and Acme2.

Example 1: Edge1 Config

Example 2: Edge2 Config


Example 3: Acme1 Config

Example 4: Acme2 Config


Answer on Paper, or Maybe Test in Lab

Next, write your answer on paper. Or if you have some real gear, or other tools, configure the lab with those tools.

To test your solution, if you happen to try it with CML/VIRL or real gear, you can verify the configuration and operation of the tunnel interface by issuing the show interfaces tunnel0 command on routers Edge1 and Edge2. If OSPF is configured correctly it should form a neighborship across the tunnel interface. Also, all four of the routers should learn about all the remote subnets in network; for instance, on routers Acme1 and Acme2, a show ip route command should list three subnets learned by OSPF once the lab is completed.

Additionally, look at show ip interface brief, which should show the tunnel interface, it’s IP address, and a state of up/up.

If you do implement this lab, also try a few ping commands. For instance, from router Acme1, the ping source G0/2 command would ping from Acme1’s G0/2 interface address to router Acme2’s G0/2 address ( That ping would test the routes to subnet and back to


Do this Lab with Cisco’s CML (Formerly VIRL)

You can do these labs on paper and still get a lot out of the lab. As an extra help, we have added files for Cisco Modeling Lab – Personal (CML-P). CML-P replaces Cisco Virtual Internet Routing Lab (VIRL) software, in effect serving as VIRL Version 2.

Below, find two files: a file useful with CML-P and another useful with VIRL. (Note that the CML-P file has a .yaml filetype, while the older VIRL file has a VIRL filetype.) Once the file is loaded, CML-P or VIRL will create a lab topology similar to this lab’s topology, with the initial configuration shown in the lab as well.

This lab’s CML file!

This lab’s VIRL file!

The CML/VIRL topology matches this lab topology exactly.

Answers: PPP over Ethernet 2
Answers: GRE Tunnel 3
By Chris July 21, 2016 13:05
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