Solved: Local DHCP server + Relay for non-answers

Daniel Graham · ‎06-26-2015

Is the following possible?

I'd like to have static DHCP leases configured on a Cisco 7200, if the router can't fulfill the client request (because no static lease exists for its MAC) then relay to an external DHCP server.

Any thoughts?

There is a feature my external dhcp server cant fulfill for my static leases but the Cisco can, if I were able to achieve the above I could save time moving DHCP platforms.

Thanks!

Markus Benz · ‎06-27-2015

Actually there is one workaround I could think of, but I would not recommend you to configure it:

Configure a dhcp pool on the router with only static leases no IP addresses in the pool that can be assigned dynamically. With that the router is only able to answer requests that have a static lease configured.

On the routed interface you configure a standard helper address to your server, so the same requests are also redirected to your DHCP server.

And in your DHCP Server, you have two options, depending on the vendor.

1) tell the server to not answer requests for the MAC addresses that should have static leases.

2) configure a delay. So that you're DHCP server is slower in answering requests then the 7200.

As said, I don't think this is a good way to go. I should rather try to fix this on the DHCP Server, but this is a way how it could work.

Regards,
Markus

View solution in original post

Markus Benz · ‎06-27-2015

Hi Daniel,

I think what you try to achieve is not possible.

You either need to answer all requests on the 7200 (also the ones without static lease) or you need to redirect all requests to a DHCP server.

What exactly is it that you want to achieve with the static lease?
Maybe there is a way to fix that in your DHCP server.

Regards,
Markus

Markus Benz · ‎06-27-2015

Actually there is one workaround I could think of, but I would not recommend you to configure it:

Configure a dhcp pool on the router with only static leases no IP addresses in the pool that can be assigned dynamically. With that the router is only able to answer requests that have a static lease configured.

On the routed interface you configure a standard helper address to your server, so the same requests are also redirected to your DHCP server.

And in your DHCP Server, you have two options, depending on the vendor.

1) tell the server to not answer requests for the MAC addresses that should have static leases.

2) configure a delay. So that you're DHCP server is slower in answering requests then the 7200.

As said, I don't think this is a good way to go. I should rather try to fix this on the DHCP Server, but this is a way how it could work.

Regards,
Markus

Daniel Graham · ‎06-27-2015

That was actually the first thing I tried but I didn't think about the delay or the blocking of static lease MAC addresses.

I don't like doing this either, but was a thought for a work around. I was hoping there was a 'clean' way of doing a dhcp server split like this.

The dhcp server doesn't have the ability to specify bootfile-name on a per static lease basis, only globally for the pool. I would use the Cisco to handle everything but my dhcp server is radius integrated and I rely on framed-pool attribute very heavily which Cisco doesn't support.

My work around is to split my pool into multiple networks so I can group devices based on the boot file name they should receive.

I appreciate the thoughts and any other input is also appreciated!

Markus Benz · ‎06-27-2015

Ok.. I understand your problem.

I would say the clean way would be to use a DHCP server that supports what you want to do.
I can't name one right now, but I guess the open source ones could be adjusted to achieve what you want.

With your current DHCP functionalities, you could try the workaround I mentioned above.
If your DHCP supports blocking MAC's I would definitely prefer this option, since it should work under all circumstances.
If not, you can try the delay. But this is not a proper solution.

Regards,
Markus

Daniel Graham · ‎06-28-2015

Thanks again for the input, I will do some testing and see what works best.

Daniel Graham · ‎07-02-2015

Appears that the router will not answer dhcp requests when the IP helper statement is present. I have played with both blocking mac addresses / time delays and taking the dhcp server offline, still no answers from router. Remove IP helper and works fine.

Looks like this is not possible?

Markus Benz · ‎07-02-2015

yes, looks like we're checkmate...

you could try to use "ip dhcp relay x.x.x.x" instead of ip helper.

And another very experimental idea is that you put another relay address which points to local broadcast address of the subnet. DHCP requests are then sent to both relay addresses.

someting like this: (assuming your local subnet is 10.0.0.0/24)

ip dhcp relay x.x.x.x (your DCHP Server)
ip dhcp relay 10.0.0.255 (your local subnet broadcast)

(same should be possible with IP helper)

Regards,
Markus