Central dhcp at flexconnect

[interfacedy]

Hi Please see the below diagram with central dhcp server. Not sure how to define the "central". When we create dynamic interface at wlc, we need to enter dhcp server ip address. Can we say this dhcp server is central dhcp server? otherwise is local dhcp server? Thank you

https://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/211325-FlexConnect-Central-DHCP-Configuration-E.html

[Scott Fella]

Just define your ip helpers on the subnets just as if they were wired.

[Arshad Safrulla]

Ok, in your case for flex AP's the most viable option would be to have the VLAN configured at the remote branch and IP helper configured directly under the Layer 3 SVI (this could be in your core switch, firewall, router at remote location). 

In case of using central dhcp for flex connect (warning; you are going to over complicate your deployment and also expose your infrastructure to some bugs). For this you need to have a working dynamic interface in the WLC and the DHCP server IP configured under that interface. If you are using central dhcp in your remote branch, it is must to have NAT/PAT enabled on that ssid.

The traffic flow will be like below.

1. Client connects to ssid and receives an IP from the same range as the dynamic interface in WLC. DHCP packets are sent over the capwap tunnel to the WLC. WLC forwards it to the DHCP server configured under it's interface.

2. Client sends traffic to AP

3. AP will NAT traffic from this IP's in to it's Management IP. (AP management IP)

4. Traffic is routed over the local network and any clients behind the AP will be seen coming from AP management. So you need to have proper upstream routing and firewall rules to allow desired connectivity. 

You must use this option only when you don't have any control over the switching/routing at remote location or dedicated VLAN not available for SSID mapping at remote location.

[interfacedy]

sorry my question is not clear. What I want to know is why we say that "Central DHCP server" in that diagram is central dhcp server instead of local dhcp server? Its because the dhcp server is close to WLC? or it is on the other side of WAN? I do not think that is reason. Maybe it is due to some configuration.

[Scott Fella]

Central DHCP means that you have dhcp server(s) in a location that serves multiple sites.  Central can mean that you have services in multiple DC's as an example that provides dhcp for a single site to hundreds of sites.

[interfacedy]

Thank you Scott. What you said is what function the central dhcp server has. what i want to know is what is difference of defining/configuring between central dhcp server and local dhcp server. In another word, why we call this dhcp server as central dhcp server instead of local dhcp server? I think its because configuration difference of two kind of dhcp server. What is the difference? 

[Scott Fella]

That is a logical definition.  If you have one site and all your resources are there, then you can say it's local or centralized.  Think about your other resources you provide your users... is your mail local or centralized?  File share local or centralized? 

A typical design would be that you don't have dhcp servers in every location.  If you have hundred or thousands of site, would you want local or centralized dhcp?  Look at it that way... it really doesn't matter from a controller point of view, because you would be using ip helper address in majority of deployments.  

[Rich R]

I think in this context central DHCP means for a WLAN which is centrally switched so DHCP server is defined on the WLC interface, whereas local DHCP server (flex local switching) is not necessarily defined at all because it will be on the local router or switch on the local vlan (wireless traffic is simply bridged to the local vlan).

Put another way - the WLC must proxy (AireOS) or relay (IOS-XE) the DHCP to a central server.