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Unable to obtain an IP address through DHCP

 

 

Introduction

 

Although IP addressing is a basic issue, the subject requires comment. If the AP or wireless clients are statically addressed, be sure that the static address is indeed in the same network as the rest of the LAN. The AP communicates with rest of the network through the IP address assigned to the BVI interface. You must be sure that the IP address is numbered similarly and has the same subnet mask. The AP is not a router and does not perform any routing functions. Therefore, never consider the AP the default gateway of a network. Do not configure the IP address of the AP on any device as the default gateway of that device.

 

Resolution

 

There are several possible explanations for this problem. Verify the following:

 

In most cases, the AP and clients obtain the IP address through DHCP. If the AP is configured to receive the IP address through DHCP but does not receive a valid response from the DHCP server, the AP does this:

 

 

  • Uses a default address of 10.0.0.1
  • Reuses the last address that the DHCP server assigned the AP
  • That the DHCP scope has not been exhausted
  • That other wired clients on the same segment can be successfully addressed dynamically
  • The characteristics and configuration of the port on the hub or switch to which the AP is connected
  • The cabling between the AP and the switch
  • The configuration of the Ethernet interface on the AP
  • The Ethernet statistics on the transmit and receive sides of both of these:

 

  1. The AP
  2. The port on the hub or switch to which the AP connects

 

Scenario

 

Clients do not Receive the IP Address from the DHCP Server

 

Solution

 

The AP can also be configured as a DHCP server in order to provide the IP addresses to the wireless clients. However, there are times when the AP is configured as a DHCP server that the wireless clients do not get the IP addresses even though the pools of IP addresses are correctly defined. One of the reasons for this is that the interface BVI IP address on the AP and IP address pool defined might not be in the same IP addressing range. If this is the case, use the ip helper command on the Ethernet interface of the AP in order to point the AP to the DHCP server on the network.

 

 

ap(config)#interface fastethernet 0

        ap(config-if)ip helper <ip address of the DHCP server>

When you configure the AP as a DHCP server, it assigns IP addresses to devices on the subnet. The devices communicate with other devices on the subnet but not beyond it. If data needs to be passed beyond the subnet, you must assign a default router. The IP address of the default router should be on the same subnet as the AP configured as the DHCP server.

 

Problem Type

 

Configure / Configuration issues

Associated but cannot pass any traffic

Device cannot associate

 

Products

 

Access point

WLAN adapters (wireless card) / ACU (Aironet Client Utility)

Base Station

 

Reference

 

Connectivity Problems—AP to Wired LAN