The following variables MUST be set on the command line
or in environment variables
                 IP_ADDRESS: The IP address for this unit
        IP_SUBNET_MASK: The subnet mask for this unit
    DEFAULT_GATEWAY: The default gateway for this unit
                TFTP_SERVER: The IP address of the server to fetch from
                      TFTP_FILE: The filename to fetch

The following variables are OPTIONAL:
             TFTP_VERBOSE: Print setting. 0=quiet, 1=progress(default), 2=verbose
    TFTP_RETRY_COUNT: Retry count for ARP and TFTP (default=18)
             TFTP_TIMEOUT: Overall timeout of operation in seconds (default=7200)
         TFTP_CHECKSUM: Perform checksum test on image, 0=no, 1=yes (default=1)

rommon B15 > boot tftp://192.168.30.111/asr9k-mini-px.vm-5.3.4

Performing tftpdnld
Mgt LAN 0 interface is selected
tftp process packet: last packet, block=17026, size=203.Image size=545587915, TFTP download size=545587915