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300-370 · Question #127

You have a single Cisco 5508 WLC with a management IP address of 172.16.24.5/24. You need to register 50 new Cisco AIR-CAP3702I-E-K9. Access Points to the WLC using DHCP option 43. The access points…

The correct answer is D. Option 43 Hex F104.AC10.1805. The APs are failing to join the WLC when DHCP option 43 is configured with an ASCII string for the WLC IP address.

Troubleshooting Wireless Infrastructure

Question

You have a single Cisco 5508 WLC with a management IP address of 172.16.24.5/24. You need to register 50 new Cisco AIR-CAP3702I-E-K9. Access Points to the WLC using DHCP option 43. The access points are connected to a switch in VLAN 37, which uses the 172.16.37.0/24 subnet. You have configured the DHCP scope on the switch like this:

Network 172.16.37.0 255.255.255.0 Default Router 172.16.37.1 Option 43 Aseii 172.16.24.5 The access points are failing to join the wireless LAN controller. Which command fixes the issue?

Options

  • AOption 43 Hex F108.CA10.1805
  • BOption 43 Hex F104.AC10.1705
  • COption 43 Hex F112.CA10.1805
  • DOption 43 Hex F104.AC10.1805

How the community answered

(57 responses)
  • A
    5% (3)
  • B
    14% (8)
  • C
    4% (2)
  • D
    77% (44)

Why each option

The APs are failing to join the WLC when DHCP option 43 is configured with an ASCII string for the WLC IP address.

AOption 43 Hex F108.CA10.1805

This option has an incorrect length byte (08 instead of 04) and an incorrect IP address conversion.

BOption 43 Hex F104.AC10.1705

This option has an incorrect IP address conversion for the third octet (17 instead of 18) and the last octet (05 is correct).

COption 43 Hex F112.CA10.1805

This option has an incorrect type (F1), length (12 instead of 04) and incorrect IP address conversion for the second octet (A0 instead of 10) and last octet (05 is correct).

DOption 43 Hex F104.AC10.1805Correct

DHCP Option 43 for Cisco APs requires a specific hexadecimal format, not a simple ASCII string of the IP address, to provide the WLC IP address. The correct hexadecimal string is built by concatenating the type (F1), length (in hex, 04 for an IP address), and the WLC IP address (172.16.24.5) converted to hexadecimal (AC.10.18.05), resulting in F104.AC10.1805.

Concept tested: DHCP Option 43 hex format for Cisco APs

Source: https://www.cisco.com/c/en/us/support/docs/wireless/aironet-1100-series-access-points/107106-dhcp-43-wlc.html

Topics

#DHCP Option 43#AP discovery#WLC joining#Hex conversion

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