BGP "no-advertise" community in practical use
I have seen situation where companies advertise some prefixes to their ISP's with "no-advertise" community in order to keep those prefixes in the ISP edge-router. At the same time, some prefixes they send over the same BGP session are without "no-advertise" community and thus spread across iBGP neighbours or the ISP and to it's eBGP neighbours as well. What is the reason behind sending some prefixes with "no-advertise" community? And what are the practical user cases for "no-advertise" community? 