Details about Band Select feature

Unanswered Question
Aug 1st, 2012

Hi Board,

currently I want to prepare a WLC test series. Now I'm stuck at the Band Select feature. I'm not clever enough to completely understand the functionality by reading the 7.2 software configuration guide or this document https://supportforums.cisco.com/docs/DOC-20630.

This is how I understand the single settings and timer.

In my examples the default settings are assumed:

- Probe cycle count: 2 (default)

- Scan Cycle Period Threshold: 200 ms (default)

- Age out dual band: 60 seconds (default)

- Blue arrows indicates usage of the 2,4 GHz band

- Blue arrows indicates usage of the 5 GHz band

Case 1: Client ignores band select

cisco_band-select_case1.JPG

- After the first (initial) probe request of the client, the probe cycle count is already increased by one (so it is "1" right now)

- Probe requests from the client, which are within the scan cycle period, are ignored - the probe cycle count is not increased (dotted line)

- After the next probe request, which is after the scan cycle period, the probe cycle count is increased by one (so it is "2" right now)

- Because the max probe cycle count is set to two (meaning that two probe requests are suppressed), the next probe request is answerd. It does not matter if the next probe request is within the scan cycle period or not, because it is not needed to increase the probe cycle count again (it is already max = 2)

Case 2: Client doesn't ignore band select

cisco_band-select_case2.JPG

- After the first (initial) probe request of the client, the probe cycle count is already increased by one (so it is "1" right now)

- Because the client does not receive an answer to the initial probe request, it tried to send the probe request in the 5 GHz band.

- The 5 GHz probe request is answered immedately.

- Then the client is associated to the SSID in the 5 GHz band. Probe requests by the client in the 2,4 GHz band within the "age out dual band" time (60 seconds) are ignored by the AP.

When does the "Age out dual band" timer actually starts decrementing? After the clients successfully associated to the 5 GHz band OR after the AP sees the first probe requests in the 5 GHz band? How does the AP know that probe requests in the 2,4 GHz and 5 GHz band come from the same client? The radio interfaces of the client have different MAC addresses! So how does the AP know, that probe requests in the 2,4 GHz band (MAC A) comes from a client, which is already associated in the 5 GHz band (MAC B)???

Are my assumptions right? Does someone have paper or a link, which describes this feature more in depth?

Best regards

Joe

Edit: Case 2 extended by the "age out dual band"

I have this problem too.
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saravlak Thu, 08/02/2012 - 15:45

Band select steer A/G client to use A and doesn't work other way.

Client will keep probing even after client associated to AP, however AP  doesn't care those probes if they're already connected to it.

When does the "Age out dual band" timer actually starts decrementing?

as soon as wireless client connected to AP on 5ghz

How does the AP know that probe requests in the 2,4 GHz and 5 GHz band come from the same client?

Wireless client uses same MAC irrespective of Radio 2.4/5ghz is used, Physical device has one MAC only, unless custom wireless driver used to create virtual MAC at software.

If there were one MAC address per radio then it is possible to connect to two different WLAN, Use one b radio to talk to AP and use the A radio for bridging.

johannes.luther Thu, 08/02/2012 - 23:06

Thank you - this partially answered my questions. AND - I feel very dumb about stating, that dual band clients have 2 different MACs (now I have a big red hand print on my forehead!!!)

Thank you for this!

So ... adjusting my inital post, the picture changes a little bit.... the start of the "age out dual band" is not at the 5 GHz probe response - it is when the client associated at 5 GHz

No the initial question is still open:

-> Case 1: Client ignores band select

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Posted August 1, 2012 at 1:02 AM
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