This is a periodic TFTP request that occurs on the network because the switch tries to auto load configuration files from a network server.
The switch can be configured to automatically download the configuration file from a TFTP server by issuing the service config command. Storing the configuration files on a TFTP server and downloading them while booting is useful when the size of the configuration file is larger than the size of the NVRAM on the switch.
When the switch is configured with this command, it generates TFTP requests to download its configuration from a TFTP server. If it is not able to find a TFTP server, or if a TFTP server is not available, the switch periodically sends TFTP requests on the network, along with an error message that starts with %SYS-4-CONFIG_RESOLVE_FAILURE: appearing on the console.