以一般的認知來說，我們都知道思科WLC可以通過如下方式完成文件的傳輸，進一步進行軟件升級。
- TFTP/FTP/SFTP
- HTTP
WLC3504上有USB port，可能會存在疑問，USB能否用來傳輸文件升級？根據官方的文檔記錄，是可以的！
但是需要注意，使用USB傳輸文件可能會命中bug，具體的bug如下：
Cisco 3504 controller cannot use USB for file transfer
CSCvh28506
Customer Visible
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Description
Symptom:
When you try to the USB mode for transfer a file to/from the WLC you see the error message:
___________________ERRORS_______________________
File Path: Attribute is Mandatory. Please specify it.
Make the necessary corrections and try again.
Even when you have a path configured on the USB method.
Conditions:
try to use USB method transfer on a WLC 3504 through GUI while not having any path configured on FTP/TFTP/SFTP transfer modes
Workaround:
configure a path in all the other methods (TFTP/FTP/SFTP) even if they are not used, after that try again to use USB method and this time should work.
or
Use CLI
https://quickview.cloudapps.cisco.com/quickview/bug/CSCvh28506
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USB support is on 8.5.135.0 (and later).
Otherwise, you need to do the following in CLI:
transfer download mode tftp transfer download path / transfer download mode http transfer download path / transfer download sftp transfer download path / transfer download ftp transfer download path /