Two locations of dhcp server with wlc 9800

The diagram isn't making it perfectly clear what the network logically looks like so I'm assuming you mean that each DHCP server only serves the VLAN it is on? So DHCP1 serves VLAN 100 and DHCP2 serves VLAN 200?

Which VLAN is the client on?
What mode is the AP in? Local or FlexConnect?

You don't need a helper address configured on the WLC if the WLC isn't the DHCP server.

Thank you very much for your fast reply!

So DHCP1 serves VLAN 100 and DHCP2 serves VLAN 200?

Yes. and the DHCP2 server is configured at sw2 itself

Which VLAN is the client on?

Now we just look at the client on vlan200

What mode is the AP in? Local or FlexConnect?

Local mode

debug ip dhcp command is added at sw2. I notice the below debug info, the ip address 10.0.20.92 below is vlan 100 svi ip address. I configured 10.0.200.10 in wlc for vlan200, but I do not know why the below message shows the client would use 10.0.20.92 as dhcp pool? Of course 10.0.20.92 is not dhcp pool address. there is no ip to offer.

07:23:56: DHCPD: DHCPDISCOVER received from client 0128.b2bd.5- through relay 10.0.20.92.
07:23:56: DHCPD: there is no address pool for 10.0.20.92.
07:24:11: DHCPD: DHCPDISCOVER received from client 0128.b2bd.5- through relay 10.0.20.92.
07:24:11: DHCPD: there is no address pool for 10.0.20.92.

Thanks.

So does VLAN 200 exist on SW1, and at the WLC?

No, not at SW1 and WLC. Vlan200 only exist at SW2. Even if we can create vlan200 at SW1, it is still needed? because the router is in the middle of the two switches. The only connection between sw1/wlc and sw2 is they can ping each other.

In Local mode, traffic from the client tunnels back to the WLC and is dropped onto VLAN 200. If VLAN 200 doesn't exist at the WLC and the switch, how will the client get an IP? You'd need to use FlexConnect for this to work if you ONLY want VLAN 200 at SW2.

The PC still cannot get ip address after changing to FlexConnect. and it still get below logging message at SW2 as before.
0128.bxxx is this PC and 10.0.20.92 is vlan100 ip address in this WLC
03:05:50: DHCPD: DHCPDISCOVER received from client 0128.bxxx through relay 10.0.20.92.
03:05:50: DHCPD: there is no address pool for 10.0.20.92.
03:05:50: DHCPD: DHCPDISCOVER received from client 0128.bxxx through relay 10.0.20.92.
03:05:50: DHCPD: there is no address pool for 10.0.20.92.

10.0.20.92 is not a DHCP server so it shouldn't be configured as a relay. In FlexConnect mode the relay would never be the WLC unless you configure the SSID to use central DHCP.

Now that you have FlexConnect configured, the client is on the same VLAN as the DHCP server, so no relay is required. If you have one configured on the WLAN, delete it.

Can you send the config for SW2 and tell me where the DHCP server sits on that switch, please?

Please see SW-2 config below. The dhcp server is sitting on SW-2

SW-2.txt

Thanks.

Do you have "service dhcp" configured on the switch?

On the WLC have you tagged the WLAN in VLAN 200? If so, the client won't be able to pass any traffic as it will be untagged when it hits the switch. If you're unsure, post the WLC config.

Also, you need to exclude the IP of the switch and the router from the DHCP server...

ip dhcp excluded-address 10.0.200.1
ip dhcp excluded-address 10.0.200.10

Select allOpen in new window

Please the config below for WLC. Note: Vlan20 in the config file is Vlan100 mentioned in the original post.

c9800show.tex

 On the WLC have you tagged the WLAN in VLAN 200?

Vlan200 should not be in WLC and SW1. Although we can see Vlan200 in the configuration file attached, it is for test purpose

Do you have "service dhcp" configured on the switch?

Yes, and its enabled by default

How can VLAN 20 in the config file be VLAN 100 in the original post? Are you saying that the WLC uses VLAN 20 but the switch uses VLAN 100? If so, it's wrong.

Nothing in the WLC config refers to VLAN 200 at all, so how are you dropping clients via FlexConnect into VLAN 200?

How can VLAN 20 in the config file be VLAN 100 in the original post? Are you saying that the WLC uses VLAN 20 but the switch uses VLAN 100?

I did several tests by creating several vlans and got something wrong. In order to make it consistent, I did not show it until now.

Nothing in the WLC config refers to VLAN 200 at all, so how are you dropping clients via FlexConnect into VLAN 200?

vlan200 exists at SW2. It might need NAT for this. Let me try it. Thank you!

The original question post and config file are hard to read. and it is pretty long. I close the question. Anyway thank you very much!