How does the CPE decide which of the APs it will connect to? I have a site where I have 4x 90* sectors in ABAB and a CPE that was west of the tower used the south facing AP. Why?
If no Preferred AP is configured for the SM, the SM will choose the AP with the best signal found during the scanning process. If the radio and authentication settings configured in the SM match the AP, the SM will register with that AP. To prevent the scenario you have described, you can populate the Preferred AP list in the SM with the information of the AP that the SM should register with. You can find additional information for the Preferred AP List functionality in the User Guide.
Sorry I should have specified - I put all 4 APs that are on this tower in the preferred ap list with their WPA keys on the SM.
The user guide doesn't go into any detail as to how it decides between them. Or if it will ever try to associate to a different one. That's what I'm asking for.
OK, no problem, I got it. The SM should prioritized the APs in the order they are listed in the Preferred AP List, so it should attempt connection with the first AP in the list first, if available. Now, that being said, I wouldn't include APs in the Preferred AP List to which you do not want the SM to ever register on. For example, in your configuration where you have ABAB channel configuration, let's say the sectors pointing North and South are using channel A while sectors East and West are using channel B. You probably do not want a SM belonging to the area covered by your North sector, registering with the South sector AP, as the SM performance will be degraded. In such a case, you should not include the South sector in the SM Preferred AP list. Same may also apply to that SM with respect to the East and West sectors. If it is obvious from the SM localization that a link with any other sector other than the North sector will result in degraded performance, it is possibly better not to include them in the list.
On the other hand, if you would rather have degraded performance vs losing connectivity to that SM completely in case of an AP failure, choose the AP that makes more sense, based on SM localization and signal strength and add that AP to the Preferred AP List, in addition to you main AP.
Finally, ePMP does not currently have a way for the SM to determine if it is connected to the most preferred AP (first in the list) or just another AP from the list. So if it connects to an AP in the list that is not the first (most preferred) one, it just remain connected to it, until the next time it drops the link and it needs to re-scan. We do have a feature in the roadmap to provide an Preferred AP Audit functionality in a near future.