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 loiface lo inet loopbackallow hotplug eth0iface eth0 inet dhcpallow-hotplug wlan0iface wlan0 inet manual    wpa-conf /etc/wpa_supplicant/wpa_supplicant_wlan0.confallow-hotplug wlan1iface wlan1 inet manual/etc/wpa_supplicant/wpa_supplicant_wlan0.conf (_wlan1 is identical):country=CActrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdevupdate_config=1network={        ssid=nexasdevnet        psk=workingatnexasisfun        key_mgmt=WPA-PSK}/etc/dhcpcd.conf has not been modifiedOutput from lsusb:Bus 001 Device 006: ID 0403:6010 Future Technology Devices International, Ltd FT2232C Dual USB-UART/FIFO ICBus 001 Device 005: ID 0bda:8178 Realtek Semiconductor Corp. RTL8192CU 802.11n WLAN AdapterBus 001 Device 004: ID 046d:c31c Logitech, Inc. Keyboard K120 for BusinessBus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp. SMSC9512/9514 Fast Ethernet AdapterBus 001 Device 002: ID 0424:9514 Standard Microsystems Corp.Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hubRelevant 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
why does ifup wlan0 also configure wlan1?
linux;networking