Lab brief & addressing
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The Mission
You're handed a network that should let PC1 reach PC2 across three routers — but it's broken in three places. Your job isn't to build; it's to diagnose, working the layers bottom-up so each show either clears a layer or corners the fault. This is where C01–C11 become a repeatable method.
You'll be able to
- Triage in order — Layer 1/2, then Layer 3, then end-to-end — instead of guessing.
- Pick the one
showcommand that confirms or kills each layer. - Name which layer a fault lived at after you fix it.
Before you start
- Finished C01–C11 (interfaces, VLANs, routing).
- Load the pre-broken lab. Three faults are planted.
Intended design
| Segment | Addressing |
|---|---|
| PC1 — SW1 (VLAN10) — R1 Gi0/0 | PC1 10.0.1.11/24 gw 10.0.1.1; R1 Gi0/0 10.0.1.1/24 |
| R1 Gi0/1 ↔ R2 Gi0/0 | 10.0.12.0/30 (.1 / .2) |
| R2 Gi0/1 ↔ R3 Gi0/0 | 10.0.23.0/30 (.1 / .2) |
| R3 Gi0/1 — PC2 | R3 Gi0/1 10.0.3.1/24; PC2 10.0.3.12/24 gw 10.0.3.1 |
The one idea behind this lab
When something can't reach something else, don't jump to the routing table. Walk up from the bottom: is the interface up/up (Layer 1)? Is it in the right VLAN / is the trunk carrying it (Layer 2)? Are the addresses and routes correct (Layer 3)? Each layer you prove good removes half the haystack. The method is slower for the first ten seconds and far faster for the next ten minutes.
- L1/L2 interface →
show ip interface brief - L2 switching →
show vlan brief,show interfaces trunk,show mac address-table - L3 routing →
show ip route, then hop-by-hopping/traceroute
See & Do Fault #1 — worked, at Layer 1
PC1 can't reach PC2. Resist the urge to look at routes. Start at the bottom on R1:
Observe
R1# show ip interface brief
Interface IP-Address OK? Method Status Protocol GigabitEthernet0/0 10.0.1.1 YES manual up up GigabitEthernet0/1 10.0.12.1 YES manual administratively down down
Gi0/1 is administratively down — the link to R2 was never enabled. The transit path is severed at Layer 1. No point checking routes that can't even leave the building.Hypothesise → test → fix → re-verify
R1# configure terminal R1(config)# interface GigabitEthernet0/1 R1(config-if)# no shutdown R1(config-if)# end
%LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up
show ip interface brief now shows Gi0/1 up/up, and ping 10.0.12.2 from R1 reaches R2. Layer 1/2 to R2 is proven good — cross it off and move up. That observe→fix→re-verify loop is the entire drill.Now You (guided) Fault #2 — at Layer 2
The transit is healthy now, but PC1 still can't even ping its own gateway 10.0.1.1. A host that can't reach its gateway is almost always a Layer 2 problem on the access side, not routing. Investigate SW1: which command shows you which VLAN PC1's port is in? Command family: show vlan brief / show interfaces status, then switchport access vlan.
Stuck? Reveal the Layer 2 diagnosis + fix
SW1# show vlan brief
VLAN Name Status Ports ---- -------------------------------- --------- ------------------------------- 1 default active Gi0/2 10 USERS active Gi0/1
PC1's port Gi0/2 is sitting in VLAN 1, but the uplink to R1 (Gi0/1) is in VLAN 10 — PC1 is stranded in the wrong broadcast domain. Fix it:
SW1(config)# interface GigabitEthernet0/2 SW1(config-if)# switchport mode access SW1(config-if)# switchport access vlan 10
Re-verify: show vlan brief shows Gi0/2 in VLAN 10, and PC1 ping 10.0.1.1 now succeeds. Layer 2 on the access side: proven good.
Solo Challenge Fault #3 — find it unaided
No commands given
PC1 now reaches R3, but ping 10.0.3.12 (PC2) still fails — and you suspect it's one-way. Localise it with the Layer 3 tools, identify the single missing/wrong route, fix it, and prove end-to-end reachability.
You pass when:
ping 10.0.3.12from PC1 succeeds, replies showing the TTL decremented for the hops crossed.- You can state, in one line each, which layer each of the three faults lived at.
Check your work
Use traceroute 10.0.3.12 from PC1 — it dies after R2. Then on R2:
R2# show ip route
C 10.0.12.0/30 is directly connected, GigabitEthernet0/0
C 10.0.23.0/30 is directly connected, GigabitEthernet0/1
S 10.0.1.0/24 [1/0] via 10.0.12.1
<no route to 10.0.3.0/24>
R2 can carry the echo toward R3's LAN only if it has a route to 10.0.3.0/24. The reply path was fine; the forward path died at R2 for that subnet. Fix:
R2(config)# ip route 10.0.3.0 255.255.255.0 10.0.23.2
The three faults: #1 R1 Gi0/1 admin-down = Layer 1; #2 PC1 port in VLAN 1 = Layer 2; #3 R2 missing route to 10.0.3.0/24 = Layer 3. One per layer — exactly why bottom-up works.
Break & Fix Why method beats guessing
Replay the start, but cheat: jump straight to Layer 3 on fault #1. You run show ip route on R1 and see the routes look fine, so you stare at ip route statements for ten minutes — while the real problem was an interface that was never no shutdown. The route to R2 was missing only because the interface was down.
up/up first." Any time you're tempted to debug routing, prove Layers 1–2 are healthy in the path first. A dead interface masquerades as a hundred different higher-layer bugs. The fix is the discipline: always start at show ip interface brief.Done check
| Command | Proof you want |
|---|---|
show ip interface brief (all routers) | every in-path interface up/up |
show vlan brief on SW1 | PC1's port and the uplink both in VLAN 10 |
show ip route on R1/R2/R3 | a complete path to every LAN, both directions |
ping 10.0.3.12 from PC1 | !!!!! end-to-end, TTL decremented per hop |