How does client dhcp work on WLC?

[jcosgrove]

I am looking for the document the explains how client DHCP work?  More specifically I am interested in how the DHCP relay function should be configured.

As of now I have the Primary and Secondary DHCP server address configured on the dynamic interfaces as well as the IP Helper statements for the DHCP servers set on the router interface to relay the DHCP requests to the DHCP server.

Do I only want the statements on the WLC dynamic interface, router interface, or both?

How does the Primary and Secondary entries work?  Do they forward to both DHCP servers at all times as helpers do or do they have some timeout protocol that will switch to the Secondary after so many failures?  If so what causes it to switch?

My DHCP servers are set up for the second server to start responding to requests if the primary fails so normally with the wired lan's we just forward all requests to both servers and the active unit will respond to the requests if it becomes active.

Also can someone please explain the whole dhcp proxy thing and why I would or would not want to use it?

Thanks.

JC

[jcosgrove]

If it helps I am running 7.0.116.0 code

[Scott Fella · Accepted solution]

This doc might help explain it. Hope this helps.

http://www.cisco.com/en/US/products/ps6366/products_tech_note09186a0080af5d13.shtml

Thanks,

Scott Fella

Sent from my iPhone

[jcosgrove]

Thanks.  Excellent doc.  Not sure why I could not find that before but it sends me in the right direction.

[Scott Fella]

It's easier than me trying to explain it... I confuse myself at times:)

Thanks,

Scott Fella

Sent from my iPhone

[jcosgrove]

Do you have an opinion on what way is better for the wireless system?  I have 8 Wism's running more then 3000 clients and have heard that it may be better to use the router with IP Helper vs. the Proxy.  Not sure why or how to determine the preferred way.

[Scott Fella]

I think you should be fine. I have a hospital with the same amount of WiSM's and using dhcp proxy. If you are not having issues you should be okay. Changing it by removing dhcp proxy and configuring ip helper is pretty easy also.

Thanks,

Scott Fella

Sent from my iPhone

[Surendra BG]

Just to add on to Scotts response!!

It all depends on what u want and how you want!!

Most of then go for Internel DHCP on the DMZ for Guest and then for Corporate it will be DHCP server.. in this case the DHCP proxy mat screw up if we dont hv the IP HELPER under the Int VLAN..

So ground Rule..

1>> For internal DHCP make sure DHCP proxy is enabled

2>> If you just have External DHCP server, DHCP proxy really dies not matter but make sure IP helper under int vlan

3>> If you have bothinternal, external DHCP, then make sure DHCP proxy is enabled and then ip helpe MUST inder vlan

HTH

Regards

Surendra

[George Stefanick]

We have 20+ WiSMs and have no issues with proxy. Only issue is when we used a FW for DHCP with our guest anchor .. In that case we had to turn off proxy.

[jcosgrove]

All of our DHCP servers are Internal and all of the dynamic vlans at this point are internal at this time.  I think we turned off the proxy some time in the past trying to troubleshoot an issue and since the IP address for the DHCP server was not understood by the engineer at the time it was disabled but that is history.

Does the proxy function solve any problem for the wireless network that the standard Helper on the router does not solve?  I see you need it for Option 82 is you use that.  Does it help the wireless controllers deal with the clients better?  Does it help lower the amount of DHCP traffic hitting the servers? 

Not trying to be difficult but I am one of those guys that needs to know how it works and why.  Not always easy to get to that level.

Thanks so far.  The information has been helpful.

[George Stefanick]

Not that I am aware. I think this was used back in the Airespace days and just carried over. I have freinds with large installs with proxy off whereby you use the ip helper.

I heard rumor that cisco was going to turn it off, but you can just do that in a new code release. If they did WLANs around the globe would break, becuase folks woud need to add ip helpers.

Perhaps the Cisco guys here have some feedback as well...