ip helper address, very simple question :)

Unanswered Question
Oct 4th, 2010

Hello,

My (temporary) router is configure like this :

interface Gi0/1

   ip address 192.168.0.1 255.255.255.0

   ip address 10.10.10.1 255.255.255.0 secondary

   ip helper-address 192.168.0.10

My DHCP server (192.168.0.10) is configure to allocate IP on the 10.10.10.0/24 network.

Is this possible to do it without a VLAN ?

At this time, DHCP try to reply with an address on the pool 192.168.0.0/24

Thanks

Jerome

I have this problem too.
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arudolph_emd Mon, 10/04/2010 - 09:20

You cold create another VLAN with the 10.10.10.0 subnet or you could add it as a secondary subnet on your existing interface in the same VLAN.  If you don't want to use another VLAN, try this:

interface Gi0/1

   ip address 10.10.10.1 255.255.255.0 secondary

Your DHCP clients may get mixed addresses between the two subnets though.  The best way to do it would be the additional VLAN.

Hope that helps.

NickNac79 Mon, 10/04/2010 - 10:27

Hi Jerome,

A couple of things - first off, you don't need to use ip helper-address in this circumstance because your DHCP server is on the same subnet as the devices it is serving addresses to.  You only need to use ip helper-address when your DHCP server is on a different subnet.  This might be simply on a different VLAN or could be on the other side of the world! :-)

Second, can you be clearer on what you are trying to accomplish?  If you want to have two DHCP scopes running on the same LAN, then the answer is no, not really.  It is possible to have one giving out addresses for one address space dynamically, and another giving out only reserved addresses, but you will still run into issues

If you can advise exactly what you;re tryig to do then we can probably help you out further.

Cheers,

Nick

Richard Burts Mon, 10/04/2010 - 11:25

Jerome

I believe that the responses from Andy and Nick both have deficiencies. The response from Andy suggests that the solution could be to configure a secondary address. But your original post clearly shows that you have configured a secondary address and so far that is not working as you want. And while I agree with Nick that you do not need to configure a helper address when the server is connected in the local subnet/VLAN he goes on to say that you can not have 2 scopes running in the same VLAN. And that is not true.

Quoting from Cisco documentation:

If the client is directly connected (the giaddr field is zero), the DHCP Server matches the DHCPDISCOVER with DHCP pool(s) that contain the subnet(s) configured on the receiving interface. If the interface has secondary IP addresses, the subnets associated with the secondary IP addresses are examined for possible allocation only after the subnet associated with the primary IP address (on the interface) is exhausted.

For more detail see this link:

http://www.cisco.com/en/US/partner/docs/ios/12_2/ip/configuration/guide/1cfdhcp.html

So you can have a scope/subnet for the primary address and a second scope/subnet for the secondary address. But the scope/subnet for the secondary address is used only when all addresses from the primary have been assigned.

HTH

Rick

jquintard@actis... Mon, 10/04/2010 - 11:38

first off, you don't need to use ip helper-address in
this circumstance because your DHCP server is on the same subnet as the devices
it is serving addresses to. 

Yes I know, it's a line I have let from my previous config with multiple vlan but It's more simple to explain my problem .

So you can have a scope/subnet for the primary address and a second scope/subnet for the secondary address. But the scope/subnet for the secondary address is used only when all addresses from the primary have been assigned.

Ok. It's probably the case with the cisco DHCP but I use the microsoft DNS, with one scope (10.10.10.0) I have no reply from the service and with two scopes (192.168.0.0 and 10.10.10.0) that work but with a reply on the bad subnet . I have added a temporary VLAN and changed my DCHP scope, it's more simple.

Thanks your help.

Jerome

Richard Burts Mon, 10/04/2010 - 12:16

Jerome

The additional information is helpful. I am not particularly expert in Microsoft DHCP but I believe that you can configure a superscope in Microsoft DHCP and it would use both subnets, though I suspect that it also would server from the primary scope until it was exhausted before it begins serving from the secondary scope.

Depending on your situation and your requirements it may be that the second VLAN is the better solution.

HTH

Rick

gatlin007 Mon, 10/04/2010 - 16:46

This is a really excellent article on DHCP and the Cisco engineer that wrote it should be commended.

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a00800f0804.shtml

Although I never advocate secondary IP addresses unless it's the only available method to negotiate a problem; this article does specifically addresses how to deal with secondary addresses in regard to DHCP:

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a00800f0804.shtml#secd

I've never used the 'ip dhcp smart-relay' command; but if my back was against the wall I suppose I'd throw it out there as I launched a counter-attack in the form of a real solution.


Chris
Airborne!

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