Quiz Entry - updated: 2026.07.14
What are the four most common reasons networks fail, and what tools do you use to troubleshoot static routing problems?
Common failures: interface down, ISP (Internet Service Provider) connection loss, link saturation, and admin configuration errors. Key tools: ping, traceroute, show ip route, show ip interface brief, and show cdp neighbors.
Why networks fail:
- Interface failure — cable unplugged, hardware failure, interface in shutdown state
- Service provider connection drop — ISP outage, WAN (Wide Area Network) link down
- Link overload/saturation — too much traffic for the link's capacity
- Administrator error — wrong next-hop IP (Internet Protocol), wrong subnet mask, missing route, typo in config
Troubleshooting commands:
| Command | What It Shows | When to Use |
|---|---|---|
ping <destination> |
Basic Layer 3 reachability | First test — can I reach it? |
ping (extended) |
Ping from a specific source interface | Test from a specific subnet's perspective |
traceroute <destination> |
Path taken, hop-by-hop, with RTT | Where does the path break? |
show ip route |
Full routing table | Does a route to the destination exist? |
show ip route <network> |
Specific route details | What path does this specific destination take? |
show ip route static |
Only static routes installed | Are my static routes active? |
show ip interface brief |
Interface status (up/down) | Are all interfaces operational? |
show cdp neighbors |
Directly connected Cisco devices | Verify Layer 1/2 connectivity to neighbors |
show running-config | include ip route |
All configured static routes | Compare configured vs. installed routes |
Tip: Compare show running-config | include ip route (what's configured) with show ip route static (what's installed). If a configured route is missing from the routing table, the exit interface is likely down or the next-hop is unreachable.
Go deeper:
traceroute (Wikipedia) — how the TTL-incrementing path trace pinpoints where forwarding breaks.