IOS - using ftp or tftp source-interface command
When using protocols such as FTP and TFTP to copy IOS image files and configurations to and from Cisco devices, the IPv4 or IPv6 address used as the source/destination of the local device is that of the exit interface through which packets are sent to reach the destination of the FTP or TFTP session based on the Routing Table of the local device.
If however, you want to specify what interface will always be used for the source/destination of such transactions, you can use the following command to define it:
ip tftp source-interface
ip ftp source-interface
For example, the command
ip tftp source-interface gigabitethernet0/1 will cause all subsequent TFTP transactions to take place with a source IP address assigned to Gi0/1. In the event that you use a loopback interface, you must ensure that the address of that interface is reachable via the established routing on the network.