WS-C4510R+E - Disable DHCP

[Electronic20]

Hi Community, 

I have a switch core model: WS-C4510R+E, we have configured several DHCP pools, we only want to deactivate the DHCP service and keep the configuration of the DHCP pools, could you tell me which command would be applied to this WS-C4510R+E device?

[Flavio Miranda · Accepted solution]

Hi @Electronic20 

 It should be

Conf t

no service dhcp

[Electronic20]

Hi @Flavio Miranda, 

Is this command "no service dhcp" recognized in the device WS-C4510R+E?

Disabling DHCP would also stop working the "ip helper-address"?

Since the DHCP servers are temporarily disconnected for maintenance and the DHCP function is temporarily configured on the WS-C4510R+E

[Flavio Miranda]

@Electronic20  reading your question again I think I did not understand your question properly. 

First, Yes, this command will disable dhcp server on 4500 and Yes, I took this commad from 4500 document

But, reading this

"Since the DHCP servers are temporarily disconnected for maintenance and the DHCP function is temporarily configured on the WS-C4510R+E"

 Actually you dont need to disable the DHCP service on the 4500, correct?

You are actually going to using the 4500 as dhcp server while you maintenance your dhcp server?

[Electronic20]

HI @Flavio Miranda  , 

It is correct, my physical server that had the DHCP function is under maintenance and we temporarily configured the DHCP on the WS-C4510R+E switch.

When we connect the physical DHCP server in production, we don't want to delete all the DHCP configuration of the WS-C4510R+E switch, just disable the DHCP.

But at the time of disabling DHCP, the "ip helper-address" command would stop working?

Since the WS-C4510R+E switch contains the vlans interfaces and will have that "ip helper-address" configuration when the physical DHCP server is in production.

[Flavio Miranda]

On this case you need to remove the "ip helper-addrress" from the interface vlan.

 The "ip helper-address" added on the itnerface vlan, make the switch look for a DHCP server. If you remove it, it will stop looking and if you create local DHCP scope, it will provide IP as soon as the request gets to the interface vlan.

[Electronic20]

I explain it graphically:

Initially it was working like this:

Now you have this operational scenario:

When the maintenance of the DHCP server is finished, it will reconnect to the WS-C4510R+E

But we don't want to remove that setting of the  WS-C4510R+E:

I would only apply the "no service dhcp" command, but the "ip helper-address" command would stop working?

[Flavio Miranda]

I got it now, thanks for the diagram.

Not, it will not.  The command "ip helper-address" is not part of the DHCP configuration on the switch. It is just a feature that tells the switch to encapsulate the DHCP request, which is a broadcast, into a unicast packet and send it out to the IP Address defined on the command. It would be a relay for DHCP request.

[Electronic20]

Thank you,

That scenario would be a manual switch.

But is there an option to configure the WS-C4510R+E switch in such a way that the physical DHCP server is the main one and the active DHCP configuration of the WS-C4510R+E switch is as backup?

So that you have the scenario of an automatic switch at the DHCP level.

[Flavio Miranda]

I dont believe is possible.  The ip helper-address is not condictional. If you add it to the interface, it wil forward the DHCP request to DHCP server, if you remove it, it will not.

[Electronic20]

Will there be the option to disable the WS-C4510R+E switch, only the DHCP pools?

I do not want to apply this "no service dhcp" command, nor do I want to unconfigure the DHCP on the WS-C4510R+E switch, since the "ip helper-adddress" does not work in my laboratory when I enable the "no service dhcp" command

[Flavio Miranda]

I beileve if you remove the helper-address is anough. And if have a lab and can test, you can check it pretty easy.

[Electronic20]

When I activate both the physical DHCP server and the WS-C4510R+E switch, the dynamic IPs are always assigned by WS-C4510R+E.

That's why I wanted to disable the dhcp service on the WS-C4510R+E without deleting its configuration, but when I disable the dhcp service the ip helper address stops working too.

In short, when I apply "no service dhcp" on WS-C4510R+E, the physical DHCP server does not assign IP even though it has the ip helper-address configured on the vlans interfaces of the WS-C4510R+E switch.

[Electronic20]

Hi @Flavio Miranda 

Thank you very much for your time, since there is no automatic option, the alternatives you mention are taken into account, regards

[MHM Cisco World · Accepted solution]

I send private message check it.