以一般的认知来说，我们都知道思科WLC可以通过如下方式完成文件的传输，进一步进行软件升级。

• TFTP/FTP/SFTP
• HTTP

WLC3504上有USB port，可能会存在疑问，USB能否用来传输文件升级？根据官方的文档记录，是可以的！

 但是需要注意，使用USB传输文件可能会命中bug，具体的bug如下：

Cisco 3504 controller cannot use USB for file transfer
CSCvh28506  
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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

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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 /