WLC 9800 - Central Switching and Central DHCP

[michalbachtin]

Hello,

we were experiencing to the issue when some WLAN clients were not able to get IP from external DHCP server.

Infrastructure: APs - WLC 9800 - Fortigate (DHCP relay) - Windows server (DHCP server)

On the WLAN policy we had:

• Central switching - ON
• Central DHCP - OFF

As far as I know the Central switching means that all traffic goes from AP to WLC via CAPWAP. So for other devices the traffic is coming from WLC.

The Central DHCP means the same, but only for DHCP traffic - so for example if we have VLAN99 then on the VLAN99 must somewhere (Fortigate) be HDCP relay.

But for us strange is that when we turned on Central DHCP then all is working properly.

I read lot of documentation but I am still lost how it is possible.

Can you explain me this, please? Thank you so much, Michal

[MHM Cisco World]

I think it mandatory for AP local mode run both central switching and central dhcp' 

You can not disable central dhcp.

MHM

[marce1000]

  - Advisory DHCP setup : https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/technical-reference/c9800-best-practices.html#DHCPbridgingandDHCPrelay

   M.
   

[michalbachtin]

Thank you for the link. I already saw the content, but did not found the relation with my question.

[ammahend]

As you said “As far as I know the Central switching means that all traffic goes from AP to WLC via CAPWAP”, all traffic includes DHCP as well, client sends DHCP discover which gets to AP. AP packs it over a capwap tunnel and sends it to controller, controller if configured as DHCP server it sends back offer, else if configured as DHCP relay then sends it forward to the relay address (DHCP Server) as unicast discovery packet 

[michalbachtin]

Thank you for your answer. Do you mean that the Central DHCP button should have no real impact in this scenario?

[Flavio Miranda]

@michalbachtin 

 what you see is the expected behavior. If you need the DHCP traffic to be send locally, turn the access point to  flexconnect mode.

[michalbachtin]

It is quite strange for me, because it has no explanation why other WLAN clients were working perfectly.

[Rich R]

> It is quite strange for me, because it has no explanation why other WLAN clients were working perfectly.
Luck <smile>
Basically like @MHM Cisco World said it's mandatory so disabling Central DHCP for that setup is an unsupported config therefore results can be unpredictable. I suspect AP got "confused" and sent some DHCP to WLC and some to local AP port.  That's a boundary condition which would never have been tested because it's unsupported.