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CWT: Wireless LAN Configuration and Troubleshooting

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Wireless LAN Configuration and Troubleshooting

Wireless LAN Configuration and Troubleshooting

This domain tests day-to-day operation: SSIDs, VLANs, DHCP, security profiles, channel and power settings, roaming, and a structured troubleshooting method.

Core configuration

  • An SSID names a network; a BSSID is a single radio's MAC address; an ESS is the set of BSSs sharing an SSID. Each SSID can map to a VLAN, and that VLAN must exist on the AP uplink and the upstream switch.
  • DHCP provides addresses; DNS resolves names; NTP keeps clocks aligned for certificates and logs. A guest SSID often uses an isolated VLAN and a captive portal.
  • Use a conservative channel plan: 20 MHz in 2.4 GHz and favor 5 GHz for capacity. Avoid maximum transmit power on every AP; lower power reduces co-channel interference and encourages roaming.
  • Enable WMM/QoS for voice and video. 802.11k/v/r assist roaming, but they must be supported consistently; an 802.11r mismatch can cause authentication failures.

Troubleshooting method

  • Define the problem: one client or many, one AP or all, one SSID or all.
  • Check association: RSSI, SNR, retries, and channel utilization. Then check authentication, VLAN, DHCP, and upstream routing.
  • Compare a working client and a failing client on the same AP; swap components only after isolating the layer.

Worked example

Users on a guest SSID associate but never receive an IP address. The AP shows strong RSSI and successful 802.11 association. The security log shows successful PSK authentication, so the issue is above Layer 2. Check the guest VLAN on the AP uplink, the switch trunk, and the DHCP scope. If the scope is exhausted or the VLAN is missing from the trunk, clients associate but fail DHCP. The reasoning: association proves RF and 802.11 authentication; DHCP failure points to VLAN, scope, or relay.

Traps

  • Hiding the SSID as a security control; it only obscures the name.
  • Blaming RF for a DHCP or VLAN fault; association without an IP is usually Layer 2 or Layer 3.
  • Using 40 MHz in 2.4 GHz and creating co-channel interference.
  • Forgetting that 802.11r, k, and v require consistent client and infrastructure support.
  • Changing many variables at once instead of isolating one layer.