While studing the controller discovery and AP states chapter in the CCNA Wireless OCG I got confused. Let me explain. These are the AP states:
I am fine with all the states except for points 2, 3 and 4. The OCG says the following:
The question is, the AP attempr to build a CAPWAP tunnel (3) with all the WLCs discovered (2)? After CAPWAP tunnels have been built, the AP selects only a WLC from the list (4)? What happens if the AP has built CAPWAP tunnels with other WLCs if they are not going to be used? According to me, this would make sense if point 3 and 4 are switched, like this:
2. WLC discovery.
3. WLC join.
4. CAPWAP tunnel.
The AP would discovery WLCs (DHCP/DNS method, broadcast, etc.), it then would join one or more WLCs, and then it would build CAPWAP tunnels only to the WLCs it wants to join (and they have accepted and replied with a CAPWAP Join Response message) in the previous step. The AP wouldn't build CAPWAP tunnels with WLCs which are not going to be used. What am I missing?
When I continue reading the OCG I reach the WLC Selection section, which confirms my guess:
From that excerpt, it seems the AP first discovers the WLCs, secondly select the WLCs it wants to join, and last it builds the corresponding DTLS/CAPWAP tunnels. Then, the order of steps 2, 3 and 4 (like Figure 11-1) showed in the previous message is wrong? Is it a mistake in the CCNA Wireless OCG?
An AP will discover the WLC via one of 4 methods:
1] Static IP at the AP (called priming if manually configured, but also stored in NVRAM if AP has previously joined WLC)
2] DHCP Option 43
3] DNS request looking for CISCO-CAPWAP-CONTROLLER (or CISCO-LWAPP-CONTROLLER if using older code)
4] Subnet broadcast (if AP and WLC are on the same L2 segment)
Once an AP discovers the WLC(s) it will attempt to join ONE WLC only. It will not have multiple CAPWAP tunnels to all candidate WLCs.
Here's a good reference:
Thanks for your replies. I am clear about the discovery process, my doubt is about the CAPWAP tunnel step. Few pages later, the OCG says again:
Tip When an AP boots and builds a list of potential controllers, it can use CAPWAP to
build a tunnel to more than one controller. The AP will join only one controller, which it
uses as the primary unit. By building a tunnel with a second controller ahead of time, before
the primary controller fails, the AP will not have to spend time building a tunnel to the
backup controller before joining it.
So I am not sure what is the correct answer? Any official documentation? The links you posted only refer to the discovery process but not the CAPWAP step.