Connectivity
What you must understand
Connectivity questions test whether you can trace a path end to end. For wired access, know that a port must be in the right VLAN, administratively up, and correctly negotiated for speed and duplex; duplex mismatch causes collisions and late collisions on one side. For trunks, both ends must allow the VLAN and agree on native VLAN. For wireless, a client must associate, authenticate, and get an IP via DHCP; failure at each step points to a different layer.
Know the troubleshooting tool ladder: check link status and interface counters, verify VLAN and trunk membership, confirm MAC learning, then check IP addressing, gateway reachability, ARP, and routing. PoE matters for access points and phones: the switch must supply sufficient power and the device class must be supported.
Roaming and controller concepts appear: a client roams when signal degrades, and fast roaming (802.11r) reduces reauthentication delay. For controller-based designs, an AP must reach its controller (or a gateway in controller-less designs) to become operational; if it cannot, it may sit in a limited state.
Traps to expect
- Blaming wireless when the client never received DHCP (Layer 3 issue).
- Forgetting that a trunk port defaulting to a different native VLAN breaks untagged traffic.
- Assuming an interface is up because the cable is plugged in; check admin state and errors.
- Ignoring PoE budget when adding APs.
- Believing a client can roam to an AP on a different subnet without proper mobility configuration.
Rule of thumb
Work bottom-up: physical link, then Layer 2 (VLAN, trunk, MAC), then Layer 3 (IP, gateway, routing), then application. Each layer has a cheap verification command.
Worked example
A new AP shows no clients. Verify the switch port is up and in the AP management VLAN, confirm PoE is delivering power, check the AP obtained an address and can reach its controller, and confirm the SSID is broadcast on that AP.