To configure DHCP for an access point (AP) on a Cisco Catalyst 3650 switch to point to a Wireless LAN Controller (WLC), you need to set up a DHCP pool and include the necessary option (Option 43) to inform the APs about the WLC's IP address.

1. Define a DHCP Pool for the APs

Configure a DHCP pool on the Catalyst 3650 to provide IP addresses to the APs.

configure terminal
ip dhcp pool AP_Pool
network <AP-NETWORK> <SUBNET-MASK>
default-router <DEFAULT-GATEWAY>
dns-server <DNS-IP>
option 43 hex <WLC-OPTION-43-HEX-VALUE>

• Replace <AP-NETWORK> and <SUBNET-MASK> with the network and subnet mask for the APs.
• Replace <DEFAULT-GATEWAY> with the gateway for the APs' VLAN.
• Replace <DNS-IP> with the DNS server IP address.
• Replace <WLC-OPTION-43-HEX-VALUE> with the hexadecimal value of the WLC IP address.

2. Calculate Option 43 Value

Option 43 provides the IP address of the WLC to the APs. The format depends on the WLC model (Cisco uses a vendor-specific format).

For Cisco:

• Convert the WLC IP address into hexadecimal.
• Prepend f104 to indicate the suboption length (4 bytes for an IPv4 address).

Example:

• WLC IP Address: 192.168.1.10
• Convert to Hexadecimal: C0.A8.01.0A
• Prepend f104: f104c0a8010a

Use f104c0a8010a as the value for option 43.

3. Bind the DHCP Pool to the VLAN

Ensure the DHCP pool is associated with the VLAN where the APs are connected.

interface vlan <VLAN-ID>
ip address <AP-NETWORK> <SUBNET-MASK>
ip helper-address <DHCP-SERVER-IP>

• Replace <VLAN-ID> with the VLAN for the APs.
• Replace <DHCP-SERVER-IP> with the Catalyst 3650’s IP (if acting as the DHCP server) or another DHCP server’s IP.

4. Enable Option 43 for Multiple WLCs (Optional)

If you have multiple WLCs, you can include their IPs in Option 43. The format changes to specify the length for each IP address.

Example for two WLCs (192.168.1.10 and 192.168.1.11):

• Convert to Hexadecimal: C0.A8.01.0A and C0.A8.01.0B
• Prepend length: f108c0a8010ac0a8010b

5. Verify Configuration

Check the DHCP configuration:

show ip dhcp pool AP_Pool
show ip dhcp binding
show run | include dhcp

Verify that the APs receive an IP address and the correct Option 43 value:

debug dhcp detail

6. Test AP to WLC Communication

Once the APs get an IP address, they should use Option 43 to discover and connect to the WLC. Verify this on the WLC by checking the AP status:

show ap summary

Troubleshooting Tips:

• Ensure the VLAN for APs is properly configured and trunked to the switch.
• Verify connectivity between the AP VLAN and the WLC.
• Use a packet capture tool to confirm the DHCP exchange includes Option 43.