When testing and debugging Wi-Fi networks it can be handy do be able to select what AP a client is connected to and maybe force a roam between AP.
Using a Linux configured with spa_supplicant this can be done using the wpa_cli tool. I often use a Raspberry PI running Rasibian for this
Start by running wpa_cli on the correct interface ie.
wpa_supplicant -i wlan0
Then to make sure wpa_supplicant knows about all Aos, run a scan
Then force a roam to the desired BSSID