Troubleshooting labstarter6 steps~12 min5 devices

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.

What you'll be able to do: Both desks reach the intranet server again, and you know the three questions that found each fault: does the LAN work, is the gateway's port up, and is each host actually pointing at the gateway.

Open the broken network in the lab

Topics: Troubleshooting · Default gateway · Interfaces · Ping

The network you're handed

Office-GW — a router, the office router and every desk's default gateway
SW-Office — a switch, the switch both desks plug into
PC-Reception — a pc, the reception desk
PC-Accounts — a pc, the accounts desk
Intranet — a server, the intranet server in the back room

Step by step

1. Reproduce the complaint from a desk
The ticket says nobody can open the intranet. Before touching anything, reproduce it from PC-Reception: ping the server, ping the gateway, then ping the other desk. Three pings, three different questions — and the pattern of answers tells you which layer to look at next.
On PC-Reception — Test the server, the gateway, and the desk next door
```
ping 10.10.10.10
ping 192.168.1.1
ping 192.168.1.11
ipconfig
```
Check: run ipconfig on PC-Reception and look for Default Gateway . . . . . . . . : 192.168.1.1.
Why: The desk-to-desk ping working proves both network cards, both cables and the switch between them — layers 1 and 2 of the LAN are fine. The gateway not answering from inside its own subnet means the fault is on the router's side of the LAN, so there is no point looking at routing yet: a packet that cannot reach the first hop never meets a routing table.
2. Find the port that is switched off
The gateway is silent, so look at both ends of the cable that leads to it. On SW-Office the uplink reads `notconnect` — no signal from the far end. On Office-GW, `show ip interface brief` gives the reason in plain words: the LAN port has its address, but it was never enabled.
On SW-Office — Check the switch end of the uplink
```
enable
show interfaces status
```
On Office-GW — Check the router end of the same cable
```
enable
show ip interface brief
show ip route
```
Check: run show ip interface brief on Office-GW and look for Gi0/0 192.168.1.1 YES manual administratively down down.
Why: `administratively down` is not a cabling fault — it means someone (or the factory default) switched the port off, and it stays off until `no shutdown`. That also explains `show ip route`: a router only installs a connected route for a port that is up, so 192.168.1.0/24 is missing and the router cannot even answer the desks.
3. Bring the gateway's LAN port up
One command under the interface fixes the first fault. Watch the switch uplink flip to `connected` and the connected route for 192.168.1.0/24 appear in `show ip route`, then repeat the ping from PC-Reception.
On Office-GW — Enable the LAN port
```
enable
configure terminal
interface Gi0/0
no shutdown
end
```
On PC-Reception — Repeat the test that failed
```
ping 192.168.1.1
ping 10.10.10.10
```
Check: run show ip interface brief on Office-GW and look for Gi0/0 192.168.1.1 YES manual up up.
Why: Status `up` means the port is enabled and has a signal; Protocol `up` means it can carry frames. Both columns up is the layer-1-and-2 all-clear, and the moment the port comes up the router installs the connected route and becomes a working gateway again.
4. One desk is still cut off — find out why
Reception works, so the router is fine now. Accounts still cannot reach the server — yet it CAN ping 192.168.1.1. A host that can reach the gateway's address but cannot get past it is not using that address as its gateway. `ipconfig` shows what it is using instead.
On PC-Accounts — Test the server, then the gateway, then read the host's settings
```
ping 10.10.10.10
ping 192.168.1.1
ipconfig
```
Check: run ipconfig on PC-Accounts and look for Default Gateway . . . . . . . . : 192.168.1.254.
Why: Pinging 192.168.1.1 from the same subnet never touches the gateway setting: the host delivers it directly. Anything off-subnet is handed to the configured gateway, and 192.168.1.254 belongs to nobody, so the host's ARP for it gets no answer and every off-subnet packet dies on the desk. The same fault kills the server's replies, because they come back to this host through that same gateway.
5. Point the accounts desk at the real gateway
Re-enter the desk's address with the correct gateway. The address and mask were fine; only the third value changes. Then ping the server again.
On PC-Accounts — Same address and mask, the router's address as the gateway
```
ipconfig Eth0 192.168.1.11 255.255.255.0 192.168.1.1
ping 10.10.10.10
```
Check: run ipconfig on PC-Accounts and look for Default Gateway . . . . . . . . : 192.168.1.1.
Why: A default gateway has to be an address that a router on the host's own subnet actually owns. There is exactly one such address on this LAN, 192.168.1.1, and every host on the segment should point at it.
6. Prove the office works from both ends
Close the ticket only after testing both directions: both desks reach the server, and the server reaches both desks. A fix you have not re-tested is a guess.
On Intranet — Test from the server side, toward both desks
```
ping 192.168.1.10
ping 192.168.1.11
```
On Office-GW — Confirm the router now knows both networks
```
enable
show ip route
```
Check: run show ip route on Office-GW and look for C 192.168.1.0/24 is directly connected, Gi0/0.
Why: Testing from the far end catches faults the near end cannot see: a host with a bad gateway can still receive, but it cannot answer. Both directions working is the proof that every hop, and every host's settings, are right.

The theory behind it

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 OSPF adjacencies — Three routers run OSPF and nothing converges. Find the area mismatch and the network statement that matches nothing.
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 complaint from a desk

2. Find the port that is switched off

3. Bring the gateway's LAN port up

4. One desk is still cut off — find out why

5. Point the accounts desk at the real gateway

6. Prove the office works from both ends

The theory behind it

More in Troubleshooting

Build it for real