acsp channel selection woes

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I am trying to use acsp and adjacent access points end up on the same channel. I also schedule a window between 12 midnight and 3am where the ap's can change channel. This does not appear to be happening.

any tips for using these features ?

Thanks
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Ben Moore

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Posted 4 years ago

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Brian Powers, Champ

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There is most likely a reason for the APs to be choosing the same channel.  At least I'd hope there is. 

If you do a "show acsp channel detail" and/or a "show acsp channel" you will see some of the variables that go into the AP selecting whatever channel is deems "clearest"  The show acsp channel is just a summary of the value assigned to each channel.  Lower the better.

The show acsp channel detail shows the variables that were used to derive this value.

Maybe these two commands will reveal what is causing the APs to choose the same channel...
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Ben Moore

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Thanks, very interesting.  As we specify use the channels 1-6-11 only on 2.4ghz.  We dont have select channel by cost enabled. I guess this must restrict the effectiveness of the ACSP ?  The AP's remain on the same channel and seems to lead to a reduction in trasmit power to compensate for neighbour ap's being on the same channel. 
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Roberto Casula, Champ

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When you specify a channel plan (e.g. 1-6-11), ACSP still works in the same way as it does if you have "Select best channel" enabled, but ACSP forcibly sets the other channels' costs (i.e. channels 2,3,4,5 etc.) to the maximum negative value so they will never be selected by the algorithm (they will always score lower than the channels in the channel plan).

ACSP will select the best channel out of 1, 6 and 11 based on the various assessment criteria.

Generally for an Enterprise deployment, especially in an environment with other networks in the area, it is best to select a channel plan (and usually 1-6-11 is best even in reg domains where channels 12, 13, 14 are available) - this is because most vendors default to 1-6-11, so chances are neighbouring businesses will be using 1-6-11 for their deployment.

There are several reasons why ACSP might select apparently "stupid" channels and/or power levels.

1. There is a high noise floor on some of the channels due to interference from external sources.
2. The cell overlap is too high - in a dense AP deployment, the first thing you should do is reduce the maximum power level in the radio profile from its default of 20dBm to a lower value. The auto-power tuning will only reduce tx power by 6dBm from this maximum value which is often insufficient and results in too much cell overlap, making channel planning impossible. It's best to start with a "sensible" power level so that the channel selection has a better chance of working from the get go.
3. The placement of APs does not lend itself to automatic channel/power selection because the AP's "view of the world" does not correspond to the clients' view of the world. This is especially true in warehouse type environments where APs are mounted maybe 20 or 30 feet high but clients are at floor-level.
4. Where the background scan cannot run because of the settings to disable background scan when clients are connected/there is traffic with voice priority/there are clients in powersave mode. Without a recent set of background scan data, ACSP has no data to work with. In this instance, a "show acsp channel-info" will display "This interface did not finish scanning all available channels"
5. There is significant interference that is triggering the "switch channel on high interference" function. This can sometimes be triggered by a client with a hardware fault that is resulting in high levels of CRC error. You can check the interface and station counters to see if that is the case.

The "show acsp channel-info", "show acsp channel-info detail" and "show acsp neighbor" commands will give you some insight into how ACSP is functioning and how the channel scores are calculated. You can also check the logs to see whether the scheduled channel selection (in your case overnight) was able to run properly.

In some situations you can tweak things so ACSP works OK. In others, you may need to enable background scanning in all circumstances and run ACSP during the day when clients are connected then once a channel plan has been define, scale back the settings. Sometimes, ACSP just doesn't work well enough in a particular environment and you need to do manual channel planning.

I have yet to find any vendor's auto tuning implementation that works in every environment. Aerohive's is, in my experience, among the better implementations.
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Ben Moore

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Thanks for the replies, very informative. You have confirmed a few points that I suspected to be the case.
I also disabled our wips profile so I can run a spectrum analysis and opted for a manual channel plan. I suspect in some of our larger sites I will opt for a manual channel plan but maybe disable the 2.4 radio on alternating adjacent access points as they are pretty dense.Thanks again for the info, good to know I am not barking up the wrong tree.
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J. Goodnough, Champ

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The auto-power tuning will only reduce tx power by 6dBm from this maximum value 
Where did you come across this information? Seems like this is the sort of thing that should be made rather obvious.
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Roberto Casula, Champ

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I don't believe I've ever seen it documented anywhere explicitly. It was something I noticed while experimenting in the lab and then later confirmed with Aerohive support.
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J. Goodnough, Champ

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That'd probably explain why I'd have problems with 2.4GhZ signal remaining too strong and causing issues while on Auto. That behavior should either be clearly documented or changed.