Disproportionate quantity of APs using channel 36

[aeccles]

We recently expanded our AP count from around 150 to 320 due to a building expansion.  After looking at some survey results, I saw a good deal of channel contention issues.  Surprisingly, 115 of my APs are using channel 36 while no other channels are used by more than 30 APs.  I confirmed all 9 of the standard channels (Non-DFS) are available in DCA, confirmed DCA was set to automatic, EDRRM is enabled, DCA channel sensitivity is set to medium. I invoked a channel update, but that didn't do much.  I just set it back to auto and set my interval to 8 hours instead of 24.  

I can see how there would be a default channel each AP starts with, then changes from when it sees the need to change.  36 makes sense.  BUT, I'm seeing APs in dense locations that are all sitting on 36.  In one instance, there are 6 aps in a room and 5 of those 6 are on ch36.  These APs are no more than 15 feet from each other and their power levels are all either 1 or 2. The next closest APs are at least 100 ft away.  My spectrum analyzer isn't picking up any interference in this area that would make the other channels off limits.  It doesn't make sense to to me that these haven't changed to another channel.

Can anyone explain this behavior?  A bug? I'm running an older version of code (8.3.150.0) because I have a few older APs I'm still in the process of replacing and need to stay on this version until that work is complete.

Thanks

[Leo Laohoo]

What APs are these?

[aeccles]

They are all 3802s. I have a mix of 3802i and 3802e and I’ll have to look and see if either one is more prone to using ch36.

[Jason Tyler]

If adding / moving AP's, its good practice to re-initialise the RRM DCA process.

Try issuing the following command on the RF group leader: (Note, make sure you do this out of working hours, as all AP's will revert to default channel, as it this builds a network neighbourhood)

config 802.11a/b channel global restart

Config Guide

https://www.cisco.com/c/en/us/td/docs/wireless/controller/8-3/config-guide/b_cg83/b_cg83_chapter_011000.html

[aeccles]

Thanks, Jason.  I'll give this a shot.  A couple of questions:

1) Is there a rule of thumb with the need to do this?  For example, if you add more than 12 APs?  If I let DCA run long enough(without the global reset), would it eventually incorporate the new APs in the same way a global restart?

2) Is the information that gets "reset" saved in the configuration, or would rebooting the controller do the same thing?

Thanks,

Aaron

[Scott Fella]

Rule of thumb is to restart rrm when adding access points. Rebooting a controller will also restart rrm. Just verify that aps are in the correct ap group if you are using that and with the right rf profile it that is being used. 36 is the default channel when aps come up, and I would also typically set DCA to the lowest, 10 minutes for a few hours to allow changes to happen.

[aeccles]

maybe this is the issue? I have 6 RF Profiles which I assume are the defaults - an "a" and "b/g" version of high, low, and typical densities.  But, I do not have any of these RF Profiles applied to my AP Groups.  Is this the cause?

[Leo Laohoo]

Agree with @Jason Tyler, TPCv2 could be the issue.

[aeccles]

Unfortunately I don't believe it is as it's been on TPCv1 until yesterday when I moved it out of desperation.  I'm switch it back now.

[Leo Laohoo]

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@aeccles wrote:

But, yes, most of them ultimately set themselves to a Tx of 1, some have a 2.

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Wait, what?

If the APs are on Tx power 1 or 2 then there is "nothing wrong" with the APs being at Channel 36.  

The bigger issue is the APs are too far apart that they could not "see" each other even when they are all on Channel 36.

[aeccles]

I follow your thinking... they wouldn't be on Tx1 and ch36 if they could see each other.  This was my thinking at first too, but once all the APs were up, the space was surveyed with Ekahau and it was screaming bloody murder about the amount of channel interference.  I confirmed by using inSSIDer as well.  If I go into one of the rooms where this is happening, it's immediately clear... From at least half of the desks in the room I have good signal (-50 or better) from at least 3 of the APs that are on 36. 

[Leo Laohoo]

Do you have any 2800?

[aeccles]

Nope, I have: 3802i, 3802e, 3602i, 1262N

[Leo Laohoo]

What is the value of the Tx power?  Is it "1"?

[aeccles]

TCP Version is set to Interference Optimal Mode (TPCv2) 

TX Power Level Assignment Method is set to Automatic

But, yes, most of them ultimately set themselves to a Tx of 1, some have a 2.