Urgent: Configure DHCP Server on inside interface ASA 5505

[ralf.rottmann]

Hi there,

We've just started with the ASA 5505. We do run a DHCP server on the inside interface, so it is in the same VLAN 1 as all of the clients. However, we cannot get it to work.

We can't use DHCP Relay, as the ASA 5505 only allows to relay to DHCP servers in a different subnet.

How can we set this up?

Or do we have to move the DHCP server to a different subnet. If so, how would we configure that scenario?

[Maykol Rojas]

What commands do you have in?

Mike

[ralf.rottmann]

Mike,

We are using ASDM 6.4. All clients are on VLAN 1 10.0.1.0/24. ASA has 10.0.1.1 and the DHCP server is on 10.0.1.2. We've configured it to be the DHCP Relay Server. If we want to enable DHCP Relay on the inside network, we get the erorr

"The DHCP Relay Agent cannot be enabled on the inside interface, because this interface is already being used to relay DHCP requests to the server."

How can we solve this?

We basically have to have the DHCP server on the same subnet as all of the clients. Any chance?

[Maykol Rojas]

Why if the clients are on the same network as the Server?

Mike

[ralf.rottmann]

Mike,

Are you saying that if our DHCP server is on the same subnet as the clients and ASA's DHCP services are disabled, the DHCP server is supposed to automatically repsond to DHCP requests from the clients?

Let me try to be precise, because I really need to get this fixed:

1. We've got ASA's outside network configured with a static IP address provided by our ISP. Works fine.

2. We've got ethernet 0/1 - 0/7 configured to be members of VLAN 1 and the inside network 10.0.1.0/24.

3. ASA has 10.0.1.1.

4. We've got an Apple Airport Extreme which provides DHCP services configured to manually use 10.0.1.2. It's connected to ehternet 0/1.

Now, a host connected on ethernet 0/2 with a manual IP address of let's say 10.0.1.100 works fine. It sees the Apple AirPort Extreme.

But: If we set the host to acquire an IP address via DHCP, it does not get an IP. It seems as if the DHCP request is not picked up by the Airport Extreme on 10.0.1.2 and not answered.

Do we have to somehow configure a rule or route or anything else for the ASA to forward DHCP requests from hosts to the Airport Extreme?

Sorry for being a pain but we really need to get this working.

[Maykol Rojas]

No, i mean its fine, but you do know that This is all layer 2 traffic. I mean, the ASA will be just like a dump switch and no configuration whatsoever is needed. I would suggest you to remove any DHCP commands from the ASA firewall as they are not needed at all.

As a test, you can have the ASA to request a DHCP address from that server and see if you get any to test the server functionality or Activate the DHCP server on the ASA itself.

Mike

[ralf.rottmann]

Thanks Mike,

That clarifies things.

May I ask you to

a) Let me know what's the best way to clear all DHCP commands fromt he ASA. Could you provide me the command?

b) Is there another command with which I can test whether the ASA can successfully request an address from the DHCP server?

Thanks in advance.

[Maykol Rojas]

Hi,

Let me know what's the best way to clear all DHCP commands fromt he ASA. Could you provide me the command?

Clear config DHCPD

Is there another command with which I can test whether the ASA can successfully request an address from the DHCP server?

Other than make the ASA to request one, no. Have you tested that server before putting it into production?

Mike

[ralf.rottmann]

Yes. The Airport was providing DHCP services just fine before we added the ASA but it also served as the router / default gateway before.

Sent from Cisco Technical Support iPhone App

[Maykol Rojas]

Did you cleared the commands for DHCP?

Mike

[ralf.rottmann]

I will try in an hour. But basically you are saying that just by having a DHCP server ion the same subnet, it should respond to requests, no extra configuration needed, correct?

Sent from Cisco Technical Support iPhone App

[Maykol Rojas]

Correct

Mike

[ralf.rottmann]

Hmm. Maybe putting the Airport Extreme into bridge mode stops the DHCP services?

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[Maykol Rojas]

Why would you put it as a bridge? Can you just modify the option of default gateway and point it to the ASA?

Mike