why does ifup wlan0 also configure wlan1?

Question

This is really strange. I'm using a raspberry Pi 3 which has builtin broadcom wireless. Works great. I added a Realtek usb wirless adapter because it has an sma connector. Installed firmware. Works great.... when the interface will come up that is.

Sometimes when I boot the Pi 3, wlan0 (realtek) will not get an IP address from dhcp. Sometimes it will. If I type ifdown wlan0/ifup wlan0 (brroadcom) wlan1 will also sometimes get an IP address.

Now this is really telling: If I bring down both wlan0 and wlan1 then type ifup wlan1 (realtek), wlan1 does not come up but wlan0 does!!!

I've always wondered how does linux knows that wlan0 is the built-in broadcom wireless while wlan1 is the Realtek wireless because I got this error in dmesg:

[    5.834507] brcmfmac: brcmf_add_if: ERROR: netdev:wlan0 already exists

/etc/network/interfaces:

auto lo
iface lo inet loopback

allow hotplug eth0
iface eth0 inet dhcp

allow-hotplug wlan0
iface wlan0 inet manual
    wpa-conf /etc/wpa_supplicant/wpa_supplicant_wlan0.conf

allow-hotplug wlan1
iface wlan1 inet manual

/etc/wpa_supplicant/wpa_supplicant_wlan0.conf (_wlan1 is identical):

country=CA
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

network={
        ssid="nexasdevnet"
        psk="workingatnexasisfun"
        key_mgmt=WPA-PSK
}

/etc/dhcpcd.conf has not been modified

Output from lsusb:

Bus 001 Device 006: ID 0403:6010 Future Technology Devices International, Ltd FT2232C Dual USB-UART/FIFO IC
Bus 001 Device 005: ID 0bda:8178 Realtek Semiconductor Corp. RTL8192CU 802.11n WLAN Adapter
Bus 001 Device 004: ID 046d:c31c Logitech, Inc. Keyboard K120 for Business
Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp. SMSC9512/9514 Fast Ethernet Adapter
Bus 001 Device 002: ID 0424:9514 Standard Microsystems Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

Relevant output from dmesg|grep usb:

[    3.109820] usb 1-1.3: new high-speed USB device number 5 using dwc_otg
[    3.211164] usb 1-1.3: New USB device found, idVendor=0bda, idProduct=8178
[    3.213327] usb 1-1.3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[    3.215399] usb 1-1.3: Product: 802.11n WLAN Adapter
[    3.217397] usb 1-1.3: Manufacturer: Realtek
[    3.219375] usb 1-1.3: SerialNumber: 00e04c000001

Answer 1

You can use the --force option to bring up wlan0.

why does ifup wlan0 also configure wlan1?

Known Limitations

The program keeps records of whether network interfaces are up or down. Under exceptional circumstances these records can become inconsistent with the real states of the interfaces. For example, an interface that was brought up using ifup and later deconfigured using ifconfig will still be recorded as up. To fix this you can use the --force option to force ifup or ifdown to run configuration or deconfiguration commands despite what it considers the current state of the interface to be.