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GRE — your routing protocol over somebody else’s network
Enterprise Advanced 5 devices 5 checked tasks 100 points Plan lab
Two sites, one provider in the middle that knows nothing about your addressing. Build a GRE tunnel across it, run OSPF inside the tunnel, and watch the provider carry traffic it cannot read. Then find out what recursive routing does to it. You build it in a browser, type real commands at each device, and every task below is
checked against the running configuration — so you find out it is wrong before anybody else does.
Open this lab in the simulator
The simulator runs in your browser with nothing to install. This lab is part of the full simulator: open it free for 7 days with a new account, then with a plan. It needs an account, which keeps your progress.
The scenario
Two sites, one provider in the middle that knows nothing about your addressing. Build a GRE tunnel across it, run OSPF inside the tunnel, and watch the provider carry traffic it cannot read. Then find out what recursive routing does to it.
Your role: a network engineer on the enterprise team. Level: Advanced Done when: all 5 goals show as passed (100 points).
Network status
Devices: 3 routers, 2 PCs, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. HQ starts with addresses (G0/0 10.1.1.1/24, G0/1 198.51.100.2/30) and static routes already configured. ISP starts with addresses (G0/0 198.51.100.1/30, G0/1 203.0.113.1/30). Branch starts with addresses (G0/0 10.2.2.1/24, G0/1 203.0.113.2/30) and static routes already configured. HQ-PC is a host with 10.1.1.10/24, gateway 10.1.1.1. BR-PC is a host with 10.2.2.10/24, gateway 10.2.2.1. Known problems
The network worked and now does not: the scenario above says what users report. The cause is a configuration fault, not a cable. Find it with show commands (interfaces, routes, neighbours, running-config) before you change anything; fix only what is wrong. Goals
Tunnel0 is up on both ends, pointed at the far public address (25 pts) The tunnel has addresses from a subnet of its own (15 pts) OSPF forms an adjacency ACROSS the tunnel (25 pts) Each site learns the other’s LAN by OSPF (20 pts) A host at one site reaches a host at the other (15 pts) The activity, in order
Tunnel0 is up on both ends, pointed at the far public address Why: Each goal is a state the lab can check on the running device. Verify with: show running-config The tunnel has addresses from a subnet of its own Why: A router interface routes nothing until it has an address and is up. Every other task builds on this one. Verify with: show ip interface brief OSPF forms an adjacency ACROSS the tunnel Why: OSPF routes only flow over a FULL adjacency. If a neighbour is stuck in INIT or EXSTART, compare the two ends’ settings. Verify with: show ip ospf neighbor Each site learns the other’s LAN by OSPF Why: The next hop must be an address on a directly connected subnet, or the route never enters the table. Verify with: show ip route A host at one site reaches a host at the other Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.2.2.10 Tips
Work in the order given: later goals depend on the earlier ones. After every change, run the verify command before moving on; do not guess. Stuck? Read the verify output first. It names the interface, neighbour or route that is wrong. Why this matters
Two sites, one provider in the middle that knows nothing about your addressing. Build a GRE tunnel across it, run OSPF inside the tunnel, and watch the provider carry traffic it cannot read. Then find out what recursive routing does to it.
What you will be able to do
Tunnel0 is up on both ends, pointed at the far public address
The tunnel has addresses from a subnet of its own
OSPF forms an adjacency ACROSS the tunnel
Each site learns the other’s LAN by OSPF
The network
ISR 2921, P Router, PC — 5 devices in all, already cabled.
Gi0/1 198.51.100.2/30 Gi0/0 198.51.100.1/30 Gi0/1 203.0.113.1/30 Gi0/1 203.0.113.2/30 Gi0/0 10.1.1.1/24 Fa0 Gi0/0 10.2.2.1/24 Fa0 HQ ISP Branch HQ-PC 10.1.1.10/24 · gw 10.1.1.1 BR-PC 10.2.2.10/24 · gw 10.2.2.1 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
HQ ISR 2921 configure it ISP P Router configure it Branch ISR 2921 configure it HQ-PC PC 10.1.1.10 / 255.255.255.0 gw 10.1.1.1 BR-PC PC 10.2.2.10 / 255.255.255.0 gw 10.2.2.1
Cabling
HQ Gi0/1 ↔ ISP Gi0/0 ISP Gi0/1 ↔ Branch Gi0/1 HQ Gi0/0 ↔ HQ-PC Fa0 Branch Gi0/0 ↔ BR-PC Fa0
What is configured at the start
These are already configured when the lab opens — including, where this is a troubleshooting lab, the fault itself:
HQ interface GigabitEthernet0/0
ip address 10.1.1.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/1
ip address 198.51.100.2 255.255.255.252
no shutdown
exit
ip route 0.0.0.0 0.0.0.0 198.51.100.1 Branch interface GigabitEthernet0/0
ip address 10.2.2.1 255.255.255.0
no shutdown
exit
interface GigabitEthernet0/1
ip address 203.0.113.2 255.255.255.252
no shutdown
exit
ip route 0.0.0.0 0.0.0.0 203.0.113.1 ISP interface GigabitEthernet0/0
ip address 198.51.100.1 255.255.255.252
no shutdown
exit
interface GigabitEthernet0/1
ip address 203.0.113.1 255.255.255.252
no shutdown
exit Tasks, and how each one is checked
Every task is decided by reading the live network — a neighbour in FULL, a ping that succeeds, a line in the running configuration — not by looking for a keyword you typed. The same command shows you what the lab sees.
# Task Pts How to verify it yourself
1 Tunnel0 is up on both ends, pointed at the far public address 25 Checked automatically against the device’s live state (tunnelUp). and Checked automatically against the device’s live state (tunnelUp). 2 The tunnel has addresses from a subnet of its own 15 On HQ: show ip interface brief — Tunnel0 has 172.16.0.1 and is up/up. and On Branch: show ip interface brief — Tunnel0 has 172.16.0.2 and is up/up. 3 OSPF forms an adjacency ACROSS the tunnel 25 On HQ: show ip ospf neighbor shows at least 1 neighbour(s) in FULL. and On Branch: show ip ospf neighbor shows at least 1 neighbour(s) in FULL. 4 Each site learns the other’s LAN by OSPF 20 On HQ: show ip route 10.2.2.0 shows next hop 172.16.0.2. and On Branch: show ip route 10.1.1.0 shows next hop 172.16.0.1. 5 A host at one site reaches a host at the other 15 On HQ-PC: ping 10.2.2.10 must succeed.
Stuck? Ask the coach
Tell the coach what you see or what you tried. It gives you a hint, not the answer, the way a senior engineer would. Included with a free trial or any plan.
Ask the coach
Reference solution
Try it yourself first — the lab checks your work either way. This is one configuration that makes every task pass, device by device:
HQ interface Tunnel0
ip address 172.16.0.1 255.255.255.252
tunnel source GigabitEthernet0/1
tunnel destination 203.0.113.2
exit
router ospf 1
router-id 1.1.1.1
network 172.16.0.0 0.0.0.3 area 0
network 10.1.1.0 0.0.0.255 area 0 Branch interface Tunnel0
ip address 172.16.0.2 255.255.255.252
tunnel source GigabitEthernet0/1
tunnel destination 198.51.100.2
exit
router ospf 1
router-id 2.2.2.2
network 172.16.0.0 0.0.0.3 area 0
network 10.2.2.0 0.0.0.255 area 0 Where this is examined
This lab practises these topics in the zNetLaB certification courses:
GRE and IPsec tunnels, LISP, VXLAN concepts — CCNP Enterprise · ENCOR · Virtualization DMVPN phases, NHRP, IPsec — CCNP Enterprise · ENARSI · VPN DMVPN phase 2/3, IPsec profiles — CCIE Enterprise Infrastructure · Transport Technologies and Solutions Secure communications: VPN, IPsec, tunnels — CompTIA Security+ · Security Architecture VPN technologies: site-to-site IPsec, DMVPN, GRE — CCNP Security Core · Security Concepts Overlays: GRE, IPsec, SD-WAN — Service Provider · VPN Services
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