5.3 - Troubleshooting connectivity and routing
Network+ N10-009 objective 5.3 covers connectivity and routing problems. A host that automatically gets a 169.254.x.x address most likely could not reach DHCP, so it self-assigned APIPA. A PC plugged into a wall jack that stays on the wrong VLAN usually has its switch port assigned to the wrong VLAN. When a PC can ping its gateway but not a host one subnet away that replies to others, the far side is often missing a return route back to the PC's subnet. Traffic that leaves by one path and returns via another can break a stateful firewall - that is asymmetric routing. You should also know default-gateway, subnet mask and DNS misconfigurations. Expect scenario questions that describe an APIPA address, a wrong-VLAN port, a one-way reachability failure or a broken stateful session and ask the most likely cause.
Auto 169.254.x.x = DHCP unreachable (APIPA). Jack works but wrong VLAN = switch port in wrong VLAN. Can ping gateway, not a subnet-away host that answers others = missing return route. Out one path, back another (breaks stateful FW) = asymmetric routing.
Practice questions
1. A host gets 169.254.x.x automatically. What most likely failed?
- DHCP could not be reached (correct answer)
- DNS resolution failed
- The default gateway is down
- The cable is crossover
169.254.x.x is APIPA, assigned when a host cannot obtain a DHCP lease. Check the DHCP server, scope exhaustion, VLAN, or the relay/switch path.
2. A PC is plugged into a wall jack but stays on the wrong VLAN. The likely cause is:
- A speed and duplex mismatch on the link
- The switch port is in the wrong VLAN (correct answer)
- An expired or unrenewed DHCP lease
- A crossover cable used by mistake
An access port assigned to the wrong VLAN puts the host in the wrong subnet, so it gets an unexpected address/gateway. Verify and correct the switchport VLAN assignment.
3. Traffic goes out one path and returns via another, breaking a stateful firewall. This is:
- Asymmetric routing (correct answer)
- A routing loop
- Split horizon
- Duplex mismatch
Asymmetric routing sends outbound and return traffic over different paths; a stateful firewall on one path never sees the full session and drops it. Align paths or state.
4. A host can ping its subnet but not other networks. Which setting is most likely wrong?
- The DNS search suffix
- The adapter MAC address
- The device hostname
- Default gateway (correct answer)
Reaching the local subnet but nothing beyond points to a missing or wrong default gateway. The gateway forwards off-subnet traffic; verify and correct it.
5. New users can't get an address though the server is up; the pool shows no free leases. This is:
- A speed/duplex mismatch
- DHCP scope exhaustion (correct answer)
- A DNS resolution failure
- A Layer 3 routing loop
DHCP scope exhaustion means all addresses in the pool are leased, so new clients get none (often APIPA). Enlarge the scope, shorten lease time, or add subnets.
6. Two hosts have the same IP and both report an address conflict. The cause is:
- A speed and duplex mismatch
- A DNS resolution error
- A duplicate/static IP conflict (correct answer)
- A failing SFP transceiver
Two devices sharing one IP (often a static set inside a DHCP range) causes conflicts and intermittent loss. Assign unique IPs or exclude statics from the DHCP scope.