When the network breaks, the engineer who fixes it first is the one with a method. Trial-and-error works on the third try. A systematic approach to localizing the fault works on the first. This lab teaches you to climb the OSI stack methodically: confirm Layer 1 (physical), confirm Layer 2 (line protocol and MAC), confirm Layer 3 (routing and ARP), and only then look at Layer 4+ symptoms. You will use the base topology with an induced fault (an interface deliberately shut on R2) and walk through diagnosing it from R1's perspective.
What you will learn
- The diagnostic order: end-to-end ping fails, then climb the OSI stack from L1 up
- The four show commands that reveal each layer's state:
show interfaces(L1+L2),show ip interface brief(L1+L2+L3 in one line),show ip arp(L2-L3 binding),show ip route(L3 forwarding decision) - How to distinguish "physical down" from "line protocol down" from "routing wrong"
- The "next-hop reachability" test that splits the fault between local and remote
- When and how to use
pingversustraceroute
What this lab does NOT cover
- Debug commands. Useful but verbose. Out of scope.
- Packet capture analysis with EPC or Wireshark. Lab nf-12 touches on this.
- Application-layer troubleshooting. We stop at Layer 3.
The scenario
A user reports: "I cannot reach 10.255.0.3 from my router. Yesterday it worked." You are sitting at R1's console. You have the base topology configured as in nf-07 (static routes between R1, R2, and R3). Someone has shut Ethernet0/1 on R2 (which is the link to R3) - but you do not know that yet. Walk through finding it.
