CCNA-Wireless Discovery Question

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Oct 31st, 2009
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I have two questions on this:

1. Several sources all describe a new, unprovisioned LAP as going through L2 LWAPP discovery, then L3 LWAPP discovery, and repeating this cycle until it gets a response from a WLC, but I've seen nothing about how long this cycling lasts. Is there an end point, or does the LAP keep cycling until the Network Engineer wakes up from his nap and notices the problem?


2. These same several sources say (now assuming that WLCs are responding) that the LAP builds a list of responding WLCs and then picks one from the list to join with. When/how does the LAP decide it has a big enough list and does its WLC selection?


Thanks


Eric Hines

Correct Answer by Rob Huffman about 7 years 4 months ago

Hi Eric,


That has always been my understanding :)


Best of luck with your studies.


Cheers!

Rob

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Rob Huffman Sun, 11/01/2009 - 08:10
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Hi Eric,


Here is the related info :)


Layer 2 LWAPP WLC Discovery Algorithm


LWAPP communication between the AP and the WLC can be in native, Layer 2 Ethernet frames. This is known as Layer 2 LWAPP mode. Although defined in the RFC draft, ***Layer 2 LWAPP mode is considered deprecated in Cisco's implementation. Only Cisco 1000 Series LAPs support Layer 2 LWAPP mode. Also, Layer 2 LWAPP mode is not supported on Cisco 2000 Series WLCs. These WLCs support only Layer 3 LWAPP mode.


This is the first method that a LAP uses to discover a WLC. The LAPs that support Layer 2 LWAPP mode broadcast a LWAPP discovery request message in a Layer 2 LWAPP frame. If there is a WLC in the network configured for Layer 2 LWAPP mode, the controller responds with a discovery response. The LAP then moves to the join phase.



The LWAPP Discovery wants as many candidate Contollers as possible. Have a look;


The LWAPP Layer 3 WLC discovery algorithm repeats until at least one WLC is found and joined.


Note: During the LWAPP Layer 3 WLC discovery, the AP **always completes all steps 1 through 5 (discovery type: broadcast, configured, OTAP, dhcp server and dns)

in this section in order to build a list of candidate WLCs. After the AP has completed the LWAPP WLC discovery steps, the AP selects a WLC from the candidate WLC list on the basis of certain criteria, and then sends that WLC an LWAPP join request.


From this good doc;


http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a00806c9e51.shtml#topic2



The LWAPP AP goes through this process on startup for Layer 3 mode:

The LAP boots and DHCPs an IP address if it was not previously assigned a static IP address.

The LAP sends discovery requests to controllers through the various discovery algorithms and builds a controller list. Essentially, the LAP learns as many management interface addresses for the controller list as possible via:

DHCP option 43 (

DNS entry for “cisco-lwapp-controller”

Management IP addresses of controllers the LAP remembers previously

A Layer 3 broadcast on the subnet

Over the air provisioning

Statically configured information


http://www.cisco.com/en/US/products/ps6366/products_tech_note09186a00808f8599.shtml#backinfo



Hope this helps!

Rob

eehinesee Sun, 11/01/2009 - 09:53
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Rob,


This answers my first question, thanks.


However, I still wonder about my second question. From the cite, and its Step 2, "Essentially, the LAP learns as many management interface addresses for the controller list as possible via:...."


When/with what decision process does the LAP decide it has "as many...addresses...as possible?" Also, does the LAP send a discovery request "to every controller on the list" as soon as the list gets an entry, or does the LAP wait until the list is fully built before sending the first discovery request?


Thanks


Eric Hines

Rob Huffman Mon, 11/02/2009 - 06:27
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Hi Eric,


Good stuff my friend!


I believe this line captures the answer to your question;


Note: During the LWAPP Layer 3 WLC discovery, the AP **always completes all steps 1 through 5 (discovery type: broadcast, configured, OTAP, dhcp server and dns)

in this section in order to build a list of candidate WLCs. **After the AP has completed the LWAPP WLC discovery steps, the AP selects a WLC from the candidate WLC list on the basis of certain criteria, and then sends that WLC an LWAPP join request.



Cheers!

Rob


eehinesee Mon, 11/02/2009 - 09:14
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Rob,


Thanks. Just to saucer and blow this, then, the AP could get one or more WLC candidates from each of the 5 steps (in an extreme case, winding up with a dozen or more candidates), and only after completing Step 5 does it select its best candidate and send a (unicast) join request to that WLC?


Thanks


Eric Hines

Correct Answer
Rob Huffman Mon, 11/02/2009 - 10:58
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Hi Eric,


That has always been my understanding :)


Best of luck with your studies.


Cheers!

Rob

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