Linux Channel Bonding

Found it.

primary

    A string (eth0, eth2, etc) specifying which slave is the
    primary device.  The specified device will always be the
    active slave while it is available.  Only when the primary is
    off-line will alternate devices be used.  This is useful when
    one slave is preferred over another, e.g., when one slave has
    higher throughput than another.

    The primary option is only valid for active-backup mode.