According to Cisco documentation for logging, the
logging source-interface command is optional. But if set up it specifies that
syslog packets contain the IPv4 or IPv6 address of a specific interface, regardless of which interface the packet uses to exit the router.
I am confused as to what it is doing and why it is optional. At first I thought it was to indicate what interface to send the logging information out of. However, the above documentation indicates that this is not the case. If it has nothing to do with what exit interface is being used to send the logging packets, then what is its purpose?