How to configure VLAN with DHCP.. What is the use of ip helper-address...

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Nov 6th, 2008

How to configure VLAN with DHCP.. What is the use of ip helper-address...

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vishwancc Thu, 11/06/2008 - 22:36

Hi,

Router(config)# ip dhcp pool mypool

Specify the network and subnet for the addresses you want to use from the pool.

Router(dhcp-config)# network 1.1.1.0 /8 Specify the DNS domain name for the clients.

Router(dhcp-config)#domain-name mydomain.comSpecify the primary and secondary DNS servers.

Router(dhcp-config)#dns-server 1.1.1.10 1.1.1.11 Specify the default router (i.e., default gateway).

Router(dhcp-config)#default-router 1.1.1.1 Specify the lease duration for the addresses you're using from the pool.

Router(dhcp-config)#lease 7 Exit Pool Configuration Mode.

Router(dhcp-config)#exit This takes you back to the global configuration prompt. Next, exclude any addresses in the pool range that you don't want to hand out.

For example, let's say that you've decided that all IP addresses up to .100 will be for static IP devices such as servers and printers. All IP addresses above .100 will be available in the pool for DHCP clients.

Here's an example of how to exclude IP addresses .100 and below:

Router(config)#ip dhcp excluded-address 1.1.1.0 1.1.1.100

If for some reason your DHCP server is located behind the router the DHCP discovery message had to cross the router to reach the server , these are broadcast messages which router will not forward until you give the command ip helper-address.

Chao

Vishwa

arianto.wibowo Thu, 11/06/2008 - 22:54

Hi Vinoth,

use the following template if you use switch as DHCP server :

ip dhcp pool VLANx

network x.x.x.x x.x.x.x

default-router x.x.x.x

dns-server x.x.x.x

lease 3

If you use another device you can use command :

"ip helper-address x.x.x.x"

Amit Singh Fri, 11/07/2008 - 00:03

Is it that you want to configure the DHCP pool for the hosts in the VLAN to get the DHCP address. If yes then use the commands/ DHCP pool configuration as posted by the poster above.

If you want to have the Vlan interface pick up the IP address via DHCP then you have to configure " ip address dhcp " under the SVI interface.

"IP helper-address" command is used to forward the DHCP requests over an if you have your DHCP server located in a different subnet / remote site.

http://www.cisco.com/en/US/docs/ios/12_3t/ip_addr/command/reference/ip1_i1gt.html#wp1169356

-amit singh

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