Troubleshooting labadvanced6 steps~20 min5 devices

Fix the OSPF adjacencies

Three routers run OSPF and nothing converges. Find the area mismatch and the network statement that matches nothing.

What you'll be able to do: Every OSPF neighbour reaches FULL, both offices learn each other's LAN, and you can tell a neighbour stuck on a mismatch from an interface that never joined OSPF in the first place.

Open the broken network in the lab

Topics: Troubleshooting · OSPF · Dynamic routing · Wildcard masks

The network you're handed

West — a router, the west office router
Core — a router, the core router between the two offices
East — a router, the east office router
PC-West — a pc, a desk in the west office
PC-East — a pc, a desk in the east office

Step by step

1. Reproduce the outage and read West's routing table
The two offices cannot reach each other. From PC-West, ping the east desk and then your own gateway. Then read West's routing table: every network West learned from OSPF would carry an `O`.
On PC-West — Test the far office, then the local gateway
```
ping 192.168.3.10
ping 192.168.1.1
```
On West — See what West knows about the network
```
enable
show ip route
```
Check: run show ip route on West and look for C 10.0.12.0/30 is directly connected, Se0/0/0.
Why: The gateway answers, so the west LAN is fine. West's table holds only its two connected networks: the serial link is up (it has a C route), yet OSPF has taught West nothing. When a link is up but a routing protocol learns nothing across it, the next question is whether the routers ever became neighbours.
2. Compare both ends of the West–Core link
Ask West for its OSPF neighbours: Core is never FULL. `show ip ospf interface` prints each OSPF interface with its area, timers and cost — run it on West and on Core and compare the two ends of 10.0.12.0/30 line by line.
On West — West's view: its neighbours and its OSPF interfaces
```
enable
show ip ospf neighbor
show ip ospf interface
```
On Core — Core's view of the same link
```
enable
show ip ospf interface
```
Check: run show ip ospf interface on West and look for Internet Address 10.0.12.1/30, Area 1.
Why: Two routers only become neighbours when their hellos agree — same subnet, same hello and dead timers, same area — and when both ends actually send hellos (not passive). Everything matches here except the area: West put its end of the link in area 1, Core put its end in area 0. Every other network statement in this design says area 0, so West's line is the typo.
3. Put West's end of the link back in area 0
Under `router ospf 1`, remove the area 1 statement and enter the link again in area 0. The adjacency should reach FULL within the same breath, and Core should learn the west LAN.
On West — Replace the area 1 network statement with the area 0 one
```
enable
configure terminal
router ospf 1
no network 10.0.12.0 0.0.0.3 area 1
network 10.0.12.0 0.0.0.3 area 0
end
```
Check: run show ip ospf neighbor on West and look for 2.2.2.2 1 FULL.
Why: With both ends in area 0 the hellos agree, the routers exchange their databases and reach FULL, and routes start to flow: Core now holds an O route to 192.168.1.0/24. FULL is the only state that carries routes — anything short of it means no routes at all.
4. East is still missing — find out why
PC-West still cannot reach the east office. On Core, `show ip ospf neighbor` lists West and nothing else — East is not stuck, it is absent. Look at East's own OSPF interfaces and the statements that were meant to enable them.
On PC-West — Retest the east office
```
ping 192.168.3.10
```
On Core — See who Core has as neighbours now
```
enable
show ip ospf neighbor
```
On East — See which of East's interfaces run OSPF, and why
```
enable
show ip ospf interface
show ip protocols
```
Check: run show ip protocols on East and look for 10.0.32.0 0.0.0.3 area 0.
Why: A missing neighbour — rather than one stuck short of FULL — means one end is not sending hellos on that link at all. East's WAN port is absent from `show ip ospf interface`, so it is not in OSPF: a `network` statement only enables OSPF on interfaces whose address it matches, and 10.0.32.0/30 matches nothing East owns. The link is 10.0.23.0/30.
5. Correct East's network statement
Remove the swapped statement and enter the real link, 10.0.23.0 with wildcard 0.0.0.3. East's Se0/0/0 joins OSPF, the Core–East adjacency forms, and the east LAN reaches West.
On East — Replace the statement that matches nothing with the one that matches Se0/0/0
```
enable
configure terminal
router ospf 1
no network 10.0.32.0 0.0.0.3 area 0
network 10.0.23.0 0.0.0.3 area 0
end
```
On PC-West — Retest across all three routers
```
ping 192.168.3.10
```
Check: run show ip route on West and look for O 192.168.3.0/24 [110/129] via 10.0.12.2, Se0/0/0.
Why: The route in West's table proves the whole chain: East now advertises its LAN, Core passes it on, and West installs it at cost 129 — two serial hops at 64 each plus the east LAN's 1.
6. Prove every adjacency and both directions
A routing fix is proven from both ends. Ping the west desk from the east desk, confirm Core lists both neighbours in FULL, and check that East learned the west LAN too.
On PC-East — Test the path from the other end
```
ping 192.168.1.10
```
On Core — Both neighbours should be FULL
```
enable
show ip ospf neighbor
```
On East — The east office's view of the network
```
enable
show ip route
```
Check: run show ip ospf neighbor on Core and look for 3.3.3.3 1 FULL.
Why: Traffic needs a route in both directions: West must know the east LAN to send, and East must know the west LAN to answer. OSPF gives every router the whole map once every adjacency is FULL, which is why the fix is checked on Core's neighbour table and on East's routes, not only with one ping.

The theory behind it

Fix the broken office — Nobody in a two-desk office can reach the intranet server. Find the two faults and fix them, one layer at a time.
Fix the VLAN that stops at the trunk — Engineering's server is unreachable and the south desk is cut off completely. Find the wrong VLAN and the trunk that drops it.
Fix the desks DHCP forgot — The clinic's desks come up with no address. Find why the DHCP server never hears them — then why their leases still go nowhere.
Fix the NAT that translates nothing — The shop's desks and tills cannot reach the internet, yet the router itself can. Find why nothing leaves translated.
Fix the branch that fails four ways — A branch depot has lost head office, and two of its hosts have faults of their own. Find all four, one layer at a time.

Build it for real

The lab walks you through these steps and ticks each one off as your network starts working.

Open in the lab

The network you're handed

Step by step

1. Reproduce the outage and read West's routing table

2. Compare both ends of the West–Core link

3. Put West's end of the link back in area 0

4. East is still missing — find out why

5. Correct East's network statement

6. Prove every adjacency and both directions

The theory behind it

More in Troubleshooting

Build it for real