Using ip-helper without using DHCP functionality

[jamesedmonds]

Hello,

I am fairly new to Cisco, and am after a bit of help.

My scenario:

We have a new domain setup on a new VLAN (3), seperate from our current infrastructure VLAN (2).

The new domain controllers provide DHCP for our new servers, and I would also like them to handle DHCP for wireless clients.

We have one DHCP scope 10.0.0.0 255.255.0.0, and I would like to assign all wireless clients an IP in the 10.0.6.0 range.

My thinking on the best way to do this, is with a DHCP policy, that looks at the relay agent information.

I would then set the ip-helper address, on the port the wireless access point is connected to on the Cisco, to point to the DHCP server.

Then for that same port, I would seb a subscriber id in the relay agent information, and use this string to set the IP assigned to that device.

Looking into doing this, it seems the Ciscos DHCP functionality has to be turned on in order to use ip-helper.

In my config, I cannot tell if DHCP is enabled or not, I can see neither "service dhcp", nor "no service dhcp" in the config.

Assuming I were to turn it on using "service dhcp", can I then leave the actual functionality turned off? i.e. turn on the DHCP service, but not have it assign IP addresses?

Also, does turning it on cause any downtime or disruption?

I think I have to run these commands:

conf t

service dhcp

interface GigabitEthernet2/40

Ip helper-address 10.0.0.1

Ip dhcp relay information option-insert

Ip dhcp relay information option subscriber-id “wireless”

I know these are probably simple questions, so please forgive my ignorance.

James

[devils_advocate]

Where does the 10.0.0.0 /16 DHCP scope reside currently?

Whoever chose to use a /16 subnet needs a slap ;)

[Richard Burts]

James

Perhaps questions may be on the simple side, but we all started at the beginning learning simple things and these forums are a good place for both simple and complex topics. So for your questions, yes you can enable the DHCP functionality on the router without having it give out IP addresses. And no it is not disruptive to enable the DHCP functionality.

One other comment I would offer is that in the Cisco configuration most of the parameters that are at their default setting do not show up when you do show running-config which is why you do not see a setting for the DHCP functionality. If you enable it and it begins to show up in the config then you know that its default was disabled. Or if you disable it and it begins to show up in the config then you know that its default was enabled.

HTH

Rick

[jamesedmonds]

Thanks guys for your replies.

I've only been in this, my first IT job, for 3 years, so am learning as I go :)

With regards to the scope, our existing network topology is our primary site using 10.1.0.0//16, with our other site connected via VPN using 10.2.0.0/16, 10.3.0.0/16 etc etc.

I don't think this is great, as it means our broadcast traffic is probably horrific, but this is what I have inherited :)

As we are re-doing our server infrastructure, I don't want to make things too complicated just yet by adding in changing the network structure at the same time.

Would you say the best thing would be to have everything on /24 subnets? I'm not a networking expert obviously, so not sure how best to organise our network.

In any case, I shall have a go at enabling DHCP functionality to see if I can achieve what I am aiming to do.

I am sure I will be back in future with more questions on network layout and configuration.

Thanks again

[Richard Burts]

James

10.1.0.0/16, 10.2.0.0/16, etc are large address spaces especially for a single subnet (DHCP scope). Most of us would prefer to design networks with smaller size vlan/subnets and /24 is a convenient (easy to use) size to work with. But it is not magic and effective networks can be designed with different size subnets.

Good luck with enabling DHCP functionality. It should go pretty smoothly. Feel free to come back with more questions. Do continue to be active in the community.

HTH

Rick

[jamesedmonds]

Hi Richard,

Not sure if you can advise on this, but when I go into config mode, select the interface, and try to set the IP helper-address, I get an invalid input error.

I have attached a screenshot. Perhaps I am using the wrong command?

Many thanks.

[devils_advocate]

Hi James

Which model of switch or router are you trying this on?

I believe you should be able to have helper addresses on subinterfaces but it may depend on which model you are using and perhaps which IOS.

Another way to do it would be to create Vlan interfaces (SVI's) and run a Trunk link between your switch and router but having not seen your topology, I am unsure if this is possible.

Thanks

[jamesedmonds]

Hi,

We are running two Catalyst 4506 switches.

I think on IOS version 12.2(31r)SGA1

I can get a screen print of "show version" if needed.

Many thanks.

[Richard Burts]

It looks to me like you are applying the helper address command on a layer 2 interface (switchport is the default and no switchport would configure it as a layer 3 interface). ip helper-address can only be configured on layer 3 interfaces. So make that interface into layer 3 and apply helper-address to it or apply helper-address to the vlan interface for the vlan that this interface belongs to.

HTH

Rick

[jamesedmonds]

Is there any additional config on the port that is required when converting it to layer 3?

At the moment, we have 3 VLANS, (2) is our current production environment, (3) will soon be our production environment, (104) our voice VLAN.

Most of our Gigabit ports are currently set as:

interface GigabitEthernet2/40
 description *** Server Access Port ***
 switchport access vlan 3
 switchport mode access
 spanning-tree portfast disable
 spanning-tree bpduguard enable
!

If I just convert this to layer 3, without doing anything extra, can I then set the IP helper-address and keep it on VLAN 3?

Many thanks.

James

[jamesedmonds]

If I am reading correctly, I would need to first convert to a layer 3 port, and then assign an IP to that port.

conf t
interface GigaBitEthernet2/40
no switchport
ip address 10.0.6.0 255.255.0.0
ip helper-address 10.0.0.1

Does that look correct?

Thanks
James

[Richard Burts]

James

That syntax does look correct. But I am not sure that you really want to do this. In your previous post you asked "If I just convert this to layer 3, without doing anything extra, can I then set the IP helper-address and keep it on VLAN 3". The key thing is that if you make this interface into layer 3 then you take it out of the vlan. If you want to keep this interface in vlan 3 then you need to keep it as a layer 2 interface.

So it looks to me like what you really want to do is to configure the helper-address on interface vlan 3. This is the layer 3 interface associated with the vlan and the helper-address configured here would process the request generated from Gig2/40 (as well as requests from other interfaces in vlan 3).

HTH

Rick

[jamesedmonds]

Thanks for the info.

I think then, if I do:

conf t
interface vlan 3
ip helper-address 10.0.0.1

That will set all VLAN 3 switchports to forward DHCP requests to that IP.

Then I can:

conf t
interface GigabitEthernet2/40
ip dhcp relay information option-insert
ip dhcp relay information option subscriber-id wireless

Which sets that port to allow option 82 in DHCP to be inserted, and sets the subscriber ID as "wireless".

Will have a crack at it in the morning and let you know my results.

James

[Richard Burts]

James

Do give it a try and let us know the results. Certainly putting the helper-address under interface vlan 3 is right. My guess is that

ip dhcp relay information option-insert
ip dhcp relay information option subscriber-id wireless

may also be layer 3 commands that go under interface vlan 3. But try them on Gig2/40 first and see what happens.

HTH

Rick

[jamesedmonds]

Well it doesn't seem to have worked unfortunately.

Vlan 3 has a helper address of 10.0.0.1, and I was able to run the commands on Ge2/40 to add subscriber id of wireless, but my DHCP server is still not assigning addresses in the correct range.

Is there any way to tell if the Cisco is in fact adding in option 82 to the DHCp requests?

Thanks.