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CCNP ENCOR lab 16 of 36

GRE tunnels over the internet

Join the Leeds and Bristol LANs across an internet provider that can't route private addresses: build a GRE tunnel, run OSPF through it, size the MTU and MSS correctly and learn why recursive routing takes tunnels down.

Run this lab

It needs Lite: its 5 nodes are set to 5 vCPU and 3,584 MB in total. That fits the free demo too.

In your workspace: Shared, then CCNP ENCOR 350-401 guided labs, then 16 - GRE tunnels over the internet

Try it free for 30 minutesRun it on Lite from £3 a day

The free demo needs no card and no email. Every lab plan includes every guided lab.

Level
Intermediate
Time
About 55 minutes
Nodes need
5 vCPU, 3,584 MB
Smallest plan
Lite

The scenario

Fernhill Freight's Bristol office has just moved from an MPLS service to a cheap internet connection from the same provider as Leeds. The provider only routes public addresses, so Bristol's 10.20.10.0/24 and Leeds' 10.10.10.0/24 can't reach each other, even though both routers can reach the internet.

Before the security team adds encryption in the next lab, you'll build the overlay: a GRE tunnel between R1 and R2, with OSPF running through it so new LANs are learned automatically.

DeviceInterfaceAddressConnects to
R1 (Leeds)Gi0/0203.0.113.2/30ISP Gi0/0
R1Gi0/110.10.10.1/24PC1
R1Lo0 / Tunnel010.255.0.1/32 / 172.16.12.1/30–
R2 (Bristol)Gi0/0203.0.113.6/30ISP Gi0/1
R2Gi0/110.20.10.1/24PC2
R2Lo0 / Tunnel010.255.0.2/32 / 172.16.12.2/30–
ISPGi0/0 / Gi0/1 / Lo0203.0.113.1/30 / 203.0.113.5/30 / 198.51.100.1/24–
PC1 / PC2eth010.10.10.10/24 / 10.20.10.10/24gateways .1

R1 and R2 already have a default route to the ISP. Don't change the ISP router.

Topology: PC1 — R1 — ISP — R2 — PC2.

What you’ll practise

  • Explain what GRE adds to a packet and why the underlay never sees the inner addresses
  • Configure a point-to-point GRE tunnel with a source interface and a destination
  • Detect a dead far end with GRE keepalives
  • Run OSPF over the tunnel without advertising the underlay, and explain recursive routing
  • Set the tunnel IP MTU and TCP MSS to avoid fragmentation

The tasks

  1. Check the underlay
  2. Build the GRE tunnel
  3. Detect a dead far end
  4. Make room for the extra headers
  5. Route the LANs through the tunnel
  6. Test end to end

The Lab Tasks pane has the scenario, these tasks, the checks to run, hints and the full solution.

Topology and devices

The network as it opens in EVE-NG, drawn from the lab’s own canvas, and the image each node runs.

Topology of GRE tunnels over the internet5 nodes, 4 links. Every link is listed as text below the diagram.R1ISPR2PC1PC2Gi0/1eth0Gi0/1eth0Gi0/0Gi0/0Gi0/0Gi0/1
5 nodes, with the interface names each link uses. On a narrow screen, scroll the diagram sideways.
Every link, as text
  • R1 Gi0/0 to ISP Gi0/0
  • R2 Gi0/0 to ISP Gi0/1
  • R1 Gi0/1 to PC1 eth0
  • R2 Gi0/1 to PC2 eth0
Nodes in GRE tunnels over the internet, with the image each runs and the vCPU and RAM it’s set to
NodeDeviceImage in Add NodeSet to
R1Cisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB
ISPCisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB
R2Cisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB
PC1VPCS hostBuilt in to EVE-NGCounted as 1 vCPU, 1,024 MB
PC2VPCS hostBuilt in to EVE-NGCounted as 1 vCPU, 1,024 MB

How to open this lab

  1. Sign in to your workspace at lab.cloudlabbox.com.

  2. In File Manager, open Shared, then the folder CCNP ENCOR 350-401 guided labs.

  3. Open 16 - GRE tunnels over the internet and start its nodes, then open Lab Tasks for the scenario and the steps.

The lab is read-only. You can start the nodes and configure the devices, but changes to the topology can’t be saved. To change it, rebuild it in your own folder.

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CloudLabBox is independent of Cisco. The lab follows exam topics and isn’t vendor training material; vendor names identify the exams and the images in the workspace.