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New Member

DHCP IP ALLOCATION

I am facing a problem while allocating the DHCP Ip to Clients.any one pl. giude me.

My DHCP Server sits in subnet A, ROuter Eth in Subnet A.ROuter has Secondary address in subnet B.

When I tried to obtain DHCP addresses for SUbnet B clients from Server the client is not getting the IP. I think DHCP forwarding needs to be enabled on ROuter. Pl. suggest the Config part that need to be done on the Router.

Thanks in advance

Srini

9 REPLIES

Re: DHCP IP ALLOCATION

hi srini

hope this link may help u .on the webpage chek out @ ip helper-address,ip forward protocol usage guidelines try the same in u r N/W setup .

http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a00801541df.html

regds

prem

New Member

Re: DHCP IP ALLOCATION

Hi Prem,

I am thinking an alternative for my problem. I got 2 physical Interfaces on my ROuter. I want to Configure Router as DHCP on one of the Router Interface. Is it recommended? or will it unnecessarily load the Router.

can you pl. get me the URL for Configuring ROuter as DHCP Server.

rgds

Srini

New Member

Re: DHCP IP ALLOCATION

If you want to have the router acting as DHCP server, the following are the configurations.

iip dhcp pool dhcp

network x.x.x.x 255.255.255.0

under the interface which is connected to the lan, give the following command

peer default ip address pool dhcp

Hope this would help you

-Deepu

Re: DHCP IP ALLOCATION

Hi,

I'm afraid it's impossible.

The DHCP server is connected via one physical card and knows it's subnet. When it receives DHCP request broadcast from the client it supposes the client being in the same subnet and offers an IP address from the same subnet. How could it recognize which client belongs to primary or secondary subnet?

The only exception is DHCP request forwarded by a router using ip helper command. The DHCP server knows the request has come from another subnet then.

But how could you force the router to forward DHCP request to the same port from which it has come? There is no source IP address in the DHCP request packet to say it has come from the secondary subnet. If you configure IP helper with the address of DHCP server on the interface the router wouldn't forward the packet back to the interface it has come from. (If it even were, the DHCP server would receive each DHCP request twice - once from the client directly, second time forwarded by the router - with unpredictable results.)

Regards,

Milan

Gold

Re: DHCP IP ALLOCATION

Hi Kalyan,

Here is a example for you -

DHCP on Cisco Router

Configure DHCP on Cisco Router

Ensure IOS 12.1(1)T or later.

A basic config:

Building configuration...

Current configuration:

!

version 12.1

service timestamps debug uptime

service timestamps log uptime

no service password-encryption

!

hostname (what ever you assign)

!

enable password (your enable password)

!

!

!

!

!

ip subnet-zero

no ip dhcp conflict logging

!

ip dhcp pool pool1

network 192.N.N.H 255.255.255.0

domain-name (your assigned domain name)

dns-server 192.N.N.H 192.N.N.H

default-router 192.N.N.H

!

!

!

!

interface Ethernet0

ip address 192.N.N.H 255.255.255.0

no cdp enable

!

interface Serial0

ip unnumbered Ethernet0

no ip mroute-cache

shutdown

!

interface Serial1

ip unnumbered Ethernet0

shutdown

!

ip classless

ip route 0.0.0.0 0.0.0.0 a.b.c.d

no ip http server

Hope this helps -

New Member

Re: DHCP IP ALLOCATION

i'll suggest you segment using VLAN and sub-interfaces on the E0 port of the Router configured as a Trunk ?

Re: DHCP IP ALLOCATION

Yes,

this would work.

If you have a DHCP server in one VLAN and configure IP helper on the subinterface in the other VLAN.

But many E0 (i.e. 10Mbps Ethernet) router interfaces are not trunking and subinterfacing capable.

I suppose the router port is not capable (why would it use secondary address if it were?).

Regards,

Milan

New Member

Re: DHCP IP ALLOCATION

Thanks Friends for all the Inputs

New Member

Re: DHCP IP ALLOCATION

Hi,

I've had the same problem , so I decided to check on MS Support page to find out if, for one time they propose the solution...Amazingly check out DHCP Superscopes...seems to solve ours problems with secondary IP Adddress and DHCP

168140 Knowledge Base

http://support.microsoft.com/default.aspx?scid=kb;en-us;169140

161571 Knowledge Base

http://support.microsoft.com/default.aspx?scid=kb;EN-US;161571

....tomorrow i'll check

SAluti and hope this helps

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