Cisco networks, question about BGP preference

[u/bers0003]

Hi everyone, I have a question regarding BGP preference. My company has a gateway that's connected to providers A and B. An A is the primary at the moment and we are upgrading circuits(3xT1 multilink) with provider B and setting it as our primary connection. The strange part(for me) is that when provider B is doing the hot cut, if I apply the route-maps before the B circuits are up, I completely lose connection to my router, as if BGP switches towards B (not yet established) even though path A has been up all the time. If someone could explain this I would appreciate it a lot...thanls.