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Triangle 29 — OSPF repair
Enterprise Intermediate 6 devices 7 checked tasks 70 points Plan lab
Scenario — Gurugram, Ajmer and Mumbai are cabled in a triangle, so every office has two ways to every other. Everything was configured last week and the network worked. Today the end offices cannot reach each other: one router has been configured in the wrong area, so it never becomes a neighbour. Find it, fix it and prove the end-to-end paths again. 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
Gurugram, Ajmer and Mumbai are cabled in a triangle, so every office has two ways to every other. Everything was configured last week and the network worked. Today the end offices cannot reach each other: one router has been configured in the wrong area, so it never becomes a neighbour. Find it, fix it and prove the end-to-end paths again.
Your role: a network engineer on the enterprise team. Level: Intermediate Done when: all 7 goals show as passed (70 points).
Network status
Devices: 3 routers, 3 PCs, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. R-Gurugram starts with addresses (G0/0 10.48.1.1/24, G0/1 172.28.28.1/30, G0/2 172.28.28.10/30) and OSPF 85 already configured. PC-Gurugram is a host with 10.48.1.10/24, gateway 10.48.1.1. R-Ajmer starts with addresses (G0/0 10.48.2.1/24, G0/1 172.28.28.2/30, G0/2 172.28.28.5/30) and OSPF 85 already configured. PC-Ajmer is a host with 10.48.2.10/24, gateway 10.48.2.1. R-Mumbai starts with addresses (G0/0 10.48.3.1/24, G0/1 172.28.28.6/30, G0/2 172.28.28.9/30) and OSPF 85 already configured. PC-Mumbai is a host with 10.48.3.10/24, gateway 10.48.3.1. Known problems
R-Mumbai has fewer OSPF neighbours than it has cables to routers, so routes are missing beyond it. No cable or interface is faulty: the fault is one line of configuration on one router. Everything else is correct and must be left alone. Goals
R-Gurugram has 2 OSPF neighbours in FULL (8 pts) R-Ajmer has 2 OSPF neighbours in FULL (8 pts) R-Mumbai has 2 OSPF neighbours in FULL (8 pts) R-Gurugram learns 10.48.3.0/24 from the protocol (10 pts) 10.48.1.10 (Gurugram) reaches 10.48.3.10 (Mumbai) (12 pts) 10.48.2.10 (Ajmer) reaches 10.48.1.10 (Gurugram) (12 pts) Every LAN reaches every other LAN (12 pts) The activity, in order
Confirm the symptom: ping the far PC from the near PC and note where it fails. 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 Follow the path hop by hop. On each router read its neighbours and its routes, starting with the one nearest the failure. 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 Compare the suspect router with its neighbour: the same protocol, the same numbers, every link covered. 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 Fix the single wrong line and nothing else. Why: If the prefix is not in the routing table, the router drops packets for it, whatever else is configured. Verify with: show ip route Retest: neighbours, routes, then pings in both directions. Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.48.3.10 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. Common mistake: A subnet mask instead of a wildcard mask in a network statement Common mistake: Different area numbers on the two ends of a link Common mistake: Leaving the LAN port active so hellos leak to hosts Why this matters
OSPF routers exchange link-state advertisements and each builds the same map of the area, so a new LAN is found everywhere without touching the other routers. Neighbours form only when area, subnet and timers agree.
What you will be able to do
Enable OSPF with a router-id and network statements
Read neighbour state and the learned routes
Find an area mismatch
The network
ISR 2911, PC — 6 devices in all, already cabled.
Fa0 Gi0/0 10.48.1.1/24 Fa0 Gi0/0 10.48.2.1/24 Fa0 Gi0/0 10.48.3.1/24 Gi0/1 172.28.28.1/30 Gi0/1 172.28.28.2/30 Gi0/2 172.28.28.5/30 Gi0/1 172.28.28.6/30 Gi0/2 172.28.28.9/30 Gi0/2 172.28.28.10/30 R-Gurugram PC-Gurugram 10.48.1.10/24 · gw 10.48.1.1 R-Ajmer PC-Ajmer 10.48.2.10/24 · gw 10.48.2.1 R-Mumbai PC-Mumbai 10.48.3.10/24 · gw 10.48.3.1 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
R-Gurugram ISR 2911 configure it PC-Gurugram PC 10.48.1.10 / 255.255.255.0 gw 10.48.1.1 R-Ajmer ISR 2911 configure it PC-Ajmer PC 10.48.2.10 / 255.255.255.0 gw 10.48.2.1 R-Mumbai ISR 2911 configure it PC-Mumbai PC 10.48.3.10 / 255.255.255.0 gw 10.48.3.1
Cabling
PC-Gurugram Fa0 ↔ R-Gurugram Gi0/0 PC-Ajmer Fa0 ↔ R-Ajmer Gi0/0 PC-Mumbai Fa0 ↔ R-Mumbai Gi0/0 R-Gurugram Gi0/1 ↔ R-Ajmer Gi0/1 R-Ajmer Gi0/2 ↔ R-Mumbai Gi0/1 R-Mumbai Gi0/2 ↔ R-Gurugram Gi0/2
What is configured at the start
These are already configured when the lab opens — including, where this is a troubleshooting lab, the fault itself:
R-Gurugram interface g0/0
ip address 10.48.1.1 255.255.255.0
no shutdown
exit
interface g0/1
ip address 172.28.28.1 255.255.255.252
no shutdown
exit
interface g0/2
ip address 172.28.28.10 255.255.255.252
no shutdown
exit
router ospf 85
router-id 1.1.1.1
network 10.48.1.0 0.0.0.255 area 0
network 172.28.28.0 0.0.0.3 area 0
network 172.28.28.8 0.0.0.3 area 0
passive-interface GigabitEthernet0/0
exit R-Ajmer interface g0/0
ip address 10.48.2.1 255.255.255.0
no shutdown
exit
interface g0/1
ip address 172.28.28.2 255.255.255.252
no shutdown
exit
interface g0/2
ip address 172.28.28.5 255.255.255.252
no shutdown
exit
router ospf 85
router-id 2.2.2.2
network 10.48.2.0 0.0.0.255 area 0
network 172.28.28.0 0.0.0.3 area 0
network 172.28.28.4 0.0.0.3 area 0
passive-interface GigabitEthernet0/0
exit R-Mumbai interface g0/0
ip address 10.48.3.1 255.255.255.0
no shutdown
exit
interface g0/1
ip address 172.28.28.6 255.255.255.252
no shutdown
exit
interface g0/2
ip address 172.28.28.9 255.255.255.252
no shutdown
exit
router ospf 85
router-id 3.3.3.3
network 10.48.3.0 0.0.0.255 area 0
network 172.28.28.4 0.0.0.3 area 1
network 172.28.28.8 0.0.0.3 area 0
passive-interface GigabitEthernet0/0
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 R-Gurugram has 2 OSPF neighbours in FULL 8 On R-Gurugram: show ip ospf neighbor shows at least 2 neighbour(s) in FULL. 2 R-Ajmer has 2 OSPF neighbours in FULL 8 On R-Ajmer: show ip ospf neighbor shows at least 2 neighbour(s) in FULL. 3 R-Mumbai has 2 OSPF neighbours in FULL 8 On R-Mumbai: show ip ospf neighbor shows at least 2 neighbour(s) in FULL. 4 R-Gurugram learns 10.48.3.0/24 from the protocol 10 On R-Gurugram: show ip route lists 10.48.3.0/24. 5 10.48.1.10 (Gurugram) reaches 10.48.3.10 (Mumbai) 12 On PC-Gurugram: ping 10.48.3.10 must succeed. 6 10.48.2.10 (Ajmer) reaches 10.48.1.10 (Gurugram) 12 On PC-Ajmer: ping 10.48.1.10 must succeed. 7 Every LAN reaches every other LAN 12 On PC-Gurugram: ping 10.48.2.10 must succeed. and On PC-Gurugram: ping 10.48.3.10 must succeed. and On PC-Ajmer: ping 10.48.1.10 must succeed. and On PC-Ajmer: ping 10.48.3.10 must succeed. and On PC-Mumbai: ping 10.48.1.10 must succeed. and On PC-Mumbai: ping 10.48.2.10 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
Common mistakes
A subnet mask instead of a wildcard mask in a network statement
Different area numbers on the two ends of a link
Leaving the LAN port active so hellos leak to hosts
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:
R-Mumbai router ospf 85
no network 172.28.28.4 0.0.0.3 area 1
network 172.28.28.4 0.0.0.3 area 0
exit Where this is examined
CCNA 3.0 OSPFv2; CCNP ENARSI OSPF.
This lab practises these topics in the zNetLaB certification courses:
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