no service dhcp and ip helper-address

[m.saufi]

Does anyone know whether issuing the "no service dhcp" will have any impact on the PCs getting their ip addressess from DHCP server x.x.x.x with default gateway having ip helper-address as x.x.x.x?

The above command to be activated in the default Cisco IOS router.

Related issue as follows : <http://www.cisco.com/warp/public/707/cisco-sa-20041110-dhcp.shtml

Any response is very much appreciated.

Thanks.

[paddyxdoyle]

Hi,

This command will disable the ability of your router to use the helper address.

In fact i have disabled this in the past when locking down a router. When i then tried at a later date to configure DHCP relay it didn't work!

HTH

Paddy

[Richard Burts]

Most of the time I think that Paddy has good opinions. But I am really surprised at this response and must disagree with it. "no service dhcp" has to do with the ability of the router to supply dhcp services from the router while "helper-address" has to do with the ability of the router to forward packets for multiple protocols (of which dhcp is one) to devices on remote networks. If "no service dhcp" really did prevent using helper address then it would prevent tftp and netbios and other protocol packets from being forwarded.

I tested it on a router. I configured "no service dhcp" and then I configured on an Ethernet interface "ip helper-address" and encountered no problem.

HTH

Rick

[m.saufi]

Rick,

After the experience I had the last several days, indeed putting "no service dhcp" will stop PCs from getting their ip addressess from the server defined in ip helper-address. Try restarting your PC(a few times) and check whether it's able to get an IP.

Having gone the workarounds recommended by Cisco, bootps only relates to dhcp service and thus disabling the ip helper-address.

Many thanks for your feedback.

Rgds,

Msaufi

[paddyxdoyle]

Hi Rick,

Thanks for the post,

My answer to the orginal post was gained from experience in the field.

Entering "no service dhcp" on a router definetely stopped me from getting an IP address from DHCP, as i mentioned i locked down a router when installing it on the network which included using "no service dhcp", then at a later date i needed to use the helper-address for clients on one of the interfaces.

It didn't work and i scratched my head for quite a while, i did some debugging, and evetually consulted the cisco documentation which led me to re-enabling "service dhcp" and then my clients could use DHCP.

"service dhcp

To enable the Cisco IOS Dynamic Host Configuration Protocol (DHCP) server and relay agent features on your router, use the service dhcp global configuration command. To disable the Cisco IOS DHCP server and relay agent features, use the no form of this command.

service dhcp

no service dhcp"

I can't speak for the rest of the protocols used by the helper address, only DHCP

Many thanks

Paddy

[CSCO11493456]

Dear Paddy,

I Strongly agree with you. You explanations help me a lot today and it works.

Thanks.

Mohammed Hussan

[VinayD]

On field while deploying ISR 4451 I did fibd that "no service dhcp" disable dhcp service eventhough "ip helper address configured on LAN int/sub-int. I replaced old router with new ISR 4451 and config inc above command, everything tested well for user machine which was already connected but after restarting user laptop it wasn't received IP from DHCP. So after enabling service dhcp problem solved

[DanMorgan59]

Hi, 

Taken from:

http://www.cisco.com/c/en/us/td/docs/switches/lan/cisco_ie3000/software/release/12-2_44_ex/configuration/guide/scg1/swdhcp82.html#wp1218221

Step 2

service dhcp

Enable the DHCP server and relay agent on your switch. By default, this feature is enabled.

So the command seems to affect all DHCP Services.

Thanks,

Daniel

[j.a.m.e.s]

I just learnt the hard way that on switching platforms 'no service dhcp' will disable relay services in addition to disabling any internal dhcp server function. This appears to be the case on IOS, IOS-XE and NX-OS (which at the time of writing has no dhcp server).

[Nevaeb]

i know that feeling, i encountered that if you issue the 'no service dhcp' command, you will have problems with ip helper-address ....  IOS-XE  [Amsterdam], Version 17.3.6