A network administrator is troubleshooting an issue where a wireless client is unable to obtain an IP address from a DHCP server. The client is connected to an Access Point (AP) operating in Local mode, which is registered to a Wireless LAN Controller (WLC). The DHCP server is located on a different VLAN than the wireless client VLAN. Which of the following is the most likely cause of the problem?
- AThe AP's radio is disabled, preventing client association.
- BThe WLC does not have an IP helper address configured for the wireless client VLAN.
- CThe wireless client is using an incorrect SSID.
- DThe AP is not correctly registered with the WLC.
Show answer & explanationAnswer & explanation
Correct answer: B. The WLC does not have an IP helper address configured for the wireless client VLAN.
In Local mode, the AP encapsulates client traffic in CAPWAP and forwards it to the WLC. The WLC then decapsulates the traffic and forwards DHCP requests from the client's VLAN. If the DHCP server is on a different VLAN, the WLC needs an IP helper address (DHCP relay) configured on the interface associated with the client's VLAN to forward DHCP requests to the server.
Why the other options are wrong
- A. If the radio was disabled, the client would not be able to associate at all.
- C. An incorrect SSID would prevent association, not just IP address acquisition.
- D. If the AP wasn't registered, the client wouldn't even be able to associate or would get a different error.
WLC DHCP Relay for Local Mode APs
When an AP is in Local mode, the WLC acts as the DHCP relay for its clients. If the DHCP server is on a different VLAN, an IP helper address must be configured on the WLC's SVI for the client's VLAN to forward DHCP requests.
- Local mode APs tunnel client traffic to the WLC via CAPWAP.
- The WLC processes client DHCP requests (decapsulates and forwards).
- IP helper addresses are crucial on the WLC for cross-VLAN DHCP communication.
Memory trick: WLC's Helper Hand: Directing DHCP to the Right Door.