WLC DHCP request

bapatsubodh · ‎11-17-2014

Hi,

In the case, when Light weight AP is broadcasting more than one SSID ( with WLC is configured with associated VLAN interface) how does the WLC differentiates DHCP requests those are coming from different SSID before forwarding to DHCP server?

Does LWAPP/802.11 frame that is received by WLC has SSID field in the header that WLC can use to forward the DHCP request.

Any link on cisco.com / or frame capture available.

Thanks in advance.

Subodh

Rasika Nayanajith · ‎11-17-2014

Hi Subodh,

If DHCP Proxy enabled on WLC, then it will use the dynamic interface map to WLAN when relaying DHCP packet to the DHCP server. In this way DHCP server known which vlan IP to be issued for the request.

Since WLC 8.0 onwards, you can have DHCP option 82 with suboption 5 to inform DHCP server which subnet IP client is required, so effectively get rid of dynamic interface requirement. Here is the 8.0 config guide details on this feature.

http://www.cisco.com/c/en/us/td/docs/wireless/controller/8-0/configuration-guide/b_cg80/b_cg80_chapter_010011000.html#concept_934E049394094702894E9CF28E47BCC1

HTH

Rasika

**** Pls rate all useful responses ****

bapatsubodh · ‎11-18-2014

Hi,

Thanks for the Information Rasika.

MAC address table (SSID <--> mac-address-of the user's PC) can decide which SSID user has joined then WLC can follow the normal process.

Thanks,

Subodh

Rasika Nayanajith · ‎11-18-2014

Here is a WLC client debug information which you can gather information about client DHCP process.(if you could take a wireshark capture on WLC connected switchport,you would see this more clearly). Since WLC acting as a DHCP relay, it will use the dynamic interface address as GIADDR field when sending traffic to DHCP server. Refer this post & look on DHCP Request packet related debug details.(x.x.x8.120 is the dynamic interface IP in given scenario)

http://mrncciew.com/2014/10/15/wlc-client-debug-part-1/

 12:42:32.432: 04:f7:e4:ea:5b:66 DHCP transmitting DHCP REQUEST (3)
 12:42:32.432: 04:f7:e4:ea:5b:66 DHCP   op: BOOTREQUEST, htype: Ethernet, hlen: 6, hops: 1
 12:42:32.432: 04:f7:e4:ea:5b:66 DHCP   xid: 0x6dc433f2 (1841574898), secs: 2, flags: 0
 12:42:32.432: 04:f7:e4:ea:5b:66 DHCP   chaddr: 04:f7:e4:ea:5b:66
 12:42:32.433: 04:f7:e4:ea:5b:66 DHCP   ciaddr: 0.0.0.0,  yiaddr: 0.0.0.0
 12:42:32.433: 04:f7:e4:ea:5b:66 DHCP   siaddr: 0.0.0.0,  giaddr: x.x.x8.120
 12:42:32.433: 04:f7:e4:ea:5b:66 DHCP   requested ip: x.x.x8.67
 12:42:32.433: 04:f7:e4:ea:5b:66 DHCP   server id: x.x.x.100  rcvd server id: 192.0.2.1
 12:42:32.433: 04:f7:e4:ea:5b:66 DHCP sending REQUEST to x.x.x8.125 (len 350, port 1, vlan 1422)
 12:42:32.433: 04:f7:e4:ea:5b:66 DHCP selecting relay 2 - control block settings:
 dhcpServer: x.x.x.100, dhcpNetmask: 0.0.0.0,
 dhcpGateway: 0.0.0.0, dhcpRelay: x.x.x8.120  VLAN: 1422
 12:42:32.433: 04:f7:e4:ea:5b:66 DHCP selected relay 2 - NONE
 12:42:32.434: 04:f7:e4:ea:5b:66 DHCP received op BOOTREPLY (2) (len 308,vlan 1422, port 1, encap 0xec00)

Here is the brief description of this step.

"The DHCP client broadcasts on the local link; the relay agent receives the broadcast and transmits it to one or more DHCP servers using unicast. The relay agent stores its own IP address in the GIADDR field of the DHCP packet. The DHCP server uses the GIADDR to determine the subnet on which the relay agent received the broadcast, and allocates an IP address on that subnet. When the DHCP server replies to the client, it sends the reply to the GIADDR address, again using unicast. The relay agent then retransmits the response on the local network."

HTH

Rasika

**** Pls rate all useful responses ****