DHCP IP Helper in Wireless

[snarayanaraju]

Hello - It is conceptual question on CAPWAP Wiress environemnt that needs your help. Thanks in advance.

Setup: I have a small office with External dedicated DHCP server, WLC 4402 and APs 3502. All these devices are connected using 3750 Layer 3 switch. Switch is configured with SVIs [SVI for WLC Management Interface, AP-Manager Interface (VLAN 10)]

Objective: Client associating  SSID in VLAN 10 should obtain IP address from DHCP Server configured with scope that belongs to VLAN 10

Question: For the clients associating with SSID (test-ssid) configured in VLAN 10, where i have to configure the DHCP-Helper IP address. Whether I have to configure it in 3750 VLAN 10 SVI or in the AP-Manager interface configured with VLAN 10.

Thanks and i hope my question is short and precise.

regards,

SAIRAM

[michael o'nan]

I'm assuming the server is in a different subnet and is why you need helper address. On VLAN 10 you would set the helper address to be the IP address of the DHCP server.

[snarayanaraju]

Hey Michael Thanks. My doubt is, in CAPWAP setup, the DHCP broadcast from the Client will pass thru the tunnel to hit the Management Interface of WLC. How the WLC will redirect the DHCP Broadcast to the external server if the DHCP Helper is not configured in the WLC.

Similarly, what is the meaning of configuring DHCP IP Helper in SVI of the switch as i assume from my understanding that the client traffic will be tunneled inside the CAPWAP Tunnel and thus the Switch in which SVI is created will not see the DHCP broadcast traffic, Am i right?

regards,SAIRAM

[George Stefanick]

Cisco WLC does DHCP in one of two ways...

1) DHCP PROXY - Enabled by default, when a client connects the WLC interface for that VLAN will UNICAST this request to the DHCP server. Under the controller interfaces this is where you put in the DHCP sever info so that the WLC can UNICAST it ..

2) DHCP PROXY DISABLE - Then you need a helper on the wired side SVI for that vlan. 

[snarayanaraju]

Hello George - Thank you. This is my understanding from your comments. Please review and comment. Thanks in advance

When a CAPWAP client send DHCP request, the DHCP request packets are tunneled inside the CAPWAP Tunnel to hit the AP-Manager Interface. In that case, we have 2 methods to handle this DHCP packet.

Method 1: DHCP PROXY ENABLED

When DHCP Proxy is enable in the WLC and the IP address of the DHCP Server is configured in the WLC itself, the WLC will proxy the DCHP request using Unicast Packet.

Method 2: DHCP PROXY DISABLED

When DHCP Proxy is disabled and if no DHCP Server IP address is configured in the WLC, the request will be forwarded to the AP-Manager's default gateway (SVI Interface of the Switch). So, SVI Interface of the Gateway has to be configured with "IP Helper address" pointing to DHCP Server.

Am i right in this conceptual understanding?

regards/ SAIRAM

[George Stefanick · Accepted solution]

See comments:

When a CAPWAP client send DHCP request, the DHCP request packets are tunneled inside the CAPWAP Tunnel to hit the AP-Manager Interface. In that case, we have 2 methods to handle this DHCP packet.

Method 1: DHCP PROXY ENABLED

When DHCP Proxy is enable in the WLC and the IP address of the DHCP Server is configured in the WLC itself, the WLC will proxy the DCHP request using Unicast Packet.

George: Yes, it will come from the controller interface on the WLC which is mapped with the WLAN. 

Method 2: DHCP PROXY DISABLED

When DHCP Proxy is disabled and if no DHCP Server IP address is configured in the WLC, the request will be forwarded to the AP-Manager's default gateway (SVI Interface of the Switch). So, SVI Interface of the Gateway has to be configured with "IP Helper address" pointing to DHCP Server.

George:

If DHCP proxy is disabled, if you fill in a DHCP server or not, it won't be used.

The request doesn't come from the AP manager. The wireless client broadcast will leave the controllers interface for that vlan (WLAN tied to controller interface) for the GW. The GW will then forward to DHCP if helper is enabled. 

Make sense?

[snarayanaraju]

Excellent. Thank you for your awesome help to get this clarified. Your time spend on this valuable to me. Thanks George

[George Stefanick]

No worries. Thanks for supporting the rating system!

[George Stefanick]

BTW -- The ap manger has one main function. Terminate AP tunnels. Has nothing to do with DHCP.