NuGet Source Unreachable問題

當在VS裏使用NuGet Package Manager Console安裝套件時，有時候會出現以下錯誤:
The source at nuget.org [https://www.nuget.org/api/v2/] is unreachable. Falling back to NuGet Local Cache at XXX.

建立新的方案，則NuGet運作正常，估計某個地方出錯了。重開VS程式或可修正，但急於當下，可直接加下-Source選項指令:
PM> Install-Package bootstrap -Source nuget.org

網上討論連結:
c# - Source unreachable when using the NuGet Package Manager Console - Stack Overflow http://stackoverflow.com/questions/24025409/source-unreachable-when-using-the-nuget-package-manager-console