DHCP server configuration on the Cisco Aironet 1100 and 1200 series AP

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Jun 22, 2009 4:40 PM
Jun 22nd, 2009


Introduction


DHCP server configuration on the Cisco Aironet 1100 and 1200 series AP

Resolution

Dynamic Host Configuration Protocol (DHCP) server capability was first available on the Cisco Aironet 1100 and 1200 series Access Points (APs) in version 12.2(11)JA1.

When an AP is configured as a DHCP server, it can provide IP addresses to an Ethernet-capable PC connected to its Ethernet port or to wireless client devices that associate to the AP.

Setting up the DHCP Server

By default, access points are configured to receive IP settings from a DHCP server on your network. You can also configure an access point to act as a DHCP server to assign IP settings to devices on both your wired and wireless LANs.

The access point becomes a mini-DHCP server by default when it is configured with factory default settings and it cannot receive IP settings from a DHCP server. As a mini-DHCP server, the access point provides up to 20 IP addresses between 10.0.0.11 and 10.0.0.30 to a PC connected to its Ethernet port and to wireless client devices configured to use no SSID, and with all security settings disabled. The mini-DHCP server feature is disabled automatically when you assign a static IP address to the access point. Because it has a console port to simplify initial setup.

Note When you configure the access point as a DHCP server, it assigns IP addresses to devices on its 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 access point configured as the DHCP server.

For detailed information on DHCP-related commands and options, refer to the Configuring DHCP chapter in the Cisco IOS IP Configuration Guide, Release 12.3. Browse to the "Configuring DHCP" chapter:

Beginning in privileged EXEC mode, follow these steps to configure an access point to provide DHCP service and specify a default router:


Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

ip dhcp excluded-address low_address [ high_address ]

Exclude the wireless device IP address from the range of addresses the  wireless device assigns. Enter the IP address in four groups of  characters, such as 10.91.6.158.

the wireless device assumes that all IP addresses in a DHCP address pool  subnet are available for assigning to DHCP clients. You must specify  the IP addresses that the DHCP Server should not assign to clients.

(Optional) To enter a range of excluded addresses, enter the address at  the low end of the range followed by the address at the high end of the  range.

Step 3 

ip dhcp pool pool_name

Create a name for the pool of IP addresses that the wireless device  assigns in response to DHCP requests, and enter DHCP configuration mode.

Step 4 

network subnet_number
[ mask | prefix-length ]

Assign the subnet number for the address pool. The wireless device assigns IP addresses within this subnet.

(Optional) Assign a subnet mask for the address pool, or specify the  number of bits that comprise the address prefix. The prefix is an  alternative way of assigning the network mask. The prefix length must be  preceded by a forward slash (/).

Step 5 

lease { days [ hours ] [ minutes ] |
infinite }

Configure the duration of the lease for IP addresses assigned by the wireless device.

days—configure the lease duration in number of days

(optional) hours—configure the lease duration in number of hours

(optional) minutes—configure the lease duration in number of minutes

infinite—set the lease duration to infinite

Step 6 

default-router address [address2 ... address 8]

Specify the IP address of the default router for DHCP clients on the  subnet. One IP address is required; however, you can specify up to eight  addresses in one command line.

Step 7 

end

Return to privileged EXEC mode.

Step 8 

show running-config

Verify your entries.

Step 9 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no form of these commands to return to default settings.

This example shows how to configure the wireless device as a DHCP server, exclude a range of IP address, and assign a default router:

AP# configure terminal

AP(config)# ip dhcp excluded-address 172.16.1.1 172.16.1.20

AP(config)# ip dhcp pool wishbone

AP(dhcp-config)# network 172.16.1.0 255.255.255.0

AP(dhcp-config)# lease 10

AP(dhcp-config)# default-router 172.16.1.1

AP(dhcp-config)# end

To configure DHCP service on the AP, refer to the Configuring DHCP Service section of Administering the Access Point.

Note:-

When you use an external DHCP server to provide IP addresses and if the DHCP server is behind a router you can use the ip helper-address  command to forward the bootp broadcast packets received on the Router interface to the DHCP server. DHCP protocol information is carried inside of BOOTP packets. To enable BOOTP broadcast forwarding for a set of clients, configure a helper address on the router interface closest to the client. The helper address should specify the address of the DHCP server. If you have multiple servers, you can configure one helper address for each server. Because BOOTP packets are forwarded by default, DHCP information can now be forwarded by the router. The DHCP server now receives broadcasts from the DHCP clients. For more information on the usage of this command read ip helper-address command.

1. ip helper-address

To enable the forwarding of User Datagram Protocol (UDP) broadcasts, including BOOTP, received on an interface, use the ip helper-address command in interface configuration mode. To disable the forwarding of broadcast packets to specific addresses, use the no form of this command.

ip helper-address [vrf name | global] address [redundancy vrg-name]

no ip helper-address [vrf name | global] address [redundancy vrg-name]

Syntax Description


vrf name

(Optional) Enables VPN routing and forwarding (VRF) instance and VRF name.

global

(Optional) Configures a global routing table.

address

Destination broadcast or host address to be used when forwarding UDP  broadcasts. There can be more than one helper address per interface.

redundancy vrg-name

(Optional) Defines the VRG group name.

2. Examples

The following example defines an address that acts as a helper address:

interface ethernet 1

ip helper-address 121.24.43.2

The following example defines an address that acts as a helper address and is associated with the VRF named red:

interface ethernet 1/0

ip helper-address vrf red 121.25.44.2

The following example defines an address that acts as a helper address and is associated with the VRG named shop:

interface ethernet 1/0

ip helper-address 121.25.45.2 redundancy shop

Problem Type

Configure / Configuration issues

Products

Access point

Reference

Administering the Access Point

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