IP Helpers dont work for secondary interface range? 1

[RFC1795]

Hi,

Does anyone know why DHCP doesn't work for IP addresse ranges that are configured as the secondary IP address.

Making the secondary range the primary solves this, but then the "new" secondary range doesn't work any longer.

Any ideas why this is so?

Thanks .. T

 

[123tech]

Slightly confused with the wording of your post. Can you put part of your config here, or give us an example with your actual ip addresses.

 

[RFC1795]

Right then ..
Its like this:

Before:
interface Ethernet0/0
ip address 10.10.10.1 255.255.255.0 secondary (A)
ip address 10.10.20.1 255.255.255.0 (B)
ip helper-address 192.168.0.1

After:
interface Ethernet0/0
ip address 10.10.20.1 255.255.255.0 secondary (C)
ip address 10.10.10.1 255.255.255.0 (D)
ip helper-address 192.168.0.1

A = Devices in range cant get DHCP IP's
B = Devices get IP addy's fine
C = Working range moved. Now devices dont get IPs
D = Devices now get IP addresses from DHCP

Thanks .. T

 

[123tech]

I have a similar config, and it works fine on my end. I'm running IOS 12.3(5a). Did you try shutting down the interface and no shut after adding the secondary interface?

 

[RFC1795]

Interesting ..

I was told by a colleague a while ago that this technically will not work and that we should know this as techie's ...

As you can see I never knew this ;-)

So the natural course of action has always been to swop them around as needed. I've now come upon a site needing DHCP in the secondary range which now means more work etc, so I decided that I needed to know why this does not work.

I have not tried a reset on the interface as yet.
If anything I will try that before changing anything.

I'm interested to know if anyone else has a secondary IP range that is working fine to DHCP servers though ..

Cheers .. T

 

[123tech]

Well maybe it shouldn't work but it does. My situation is slightly different to a certain extent. Both of those subnets are on the same vlan, which is the default of 1. We are in the process of setting up Vlan, and the other subnets which are on other vlans (ex. Vlan7, Vlan 8, etc.) do have the ip helper command. But the one that has the secondary ip address isn't on a seperate VLAN.

interface ethernet 0/0
ip address 172.16.2.1
ip address 172.16.4.1 (secondary)

There is no ip helper command, and we created a DHCP scope for the 172.16.2.0 and 172.16.4.0 network, and it's all good.

Does this help?

 

[vipergg]

It generally will not work on the secondary as the dhcp broadcast uses only the primary address for it's broadcast , if you have 2 it doesn't know where to send the dhcp reply when it comes back . I'm not a dhcp server guy but I read you can do a superscope on the server to address this , there is nothing on the router side that can be done .

 

[RFC1795]

Thanks vipergg ... that makes sense to me. So the DHCP server needs a kicking then ..

Also 123tech .. thanks for reading and trying to assist with my questions

Cheers .. T

 

[Jynxx]

Not certain why you would want to have 2 different class C addresses on the same interface...not really certain what that is netting you.

The simplest idea, for instance if you had 2 class C networks divided in to 2 seperate port-based VLANs on the LAN, would be to put one IP on one Ethernet interface and the 2nd IP on the other Ethernet interface, then put an IP helper address on both.

Now...if the reason for your 2 class C ranges is that you have too many devices for one class C, then the best way to make it work would be to use 2 contiguous class C ranges that would fall under the same subnet mask.

An example would be 10.10.10.0 as your network, with 255.255.254.0 as your subnet mask. This allows all IP's between 10.10.10.1 and 10.10.11.254 to be in the same broadcast network, thus being able to use ONE IP helper, and ONE default gateway, such as 10.10.10.1 that you listed above. In this example the DHCP scope would be set up as a superscope.