Let me add something that could help you
the PIC add the two primary/backup paths to the router with CEF for fast switchover
the problem is when PE1 advertise best route PE2 will not send its advertised route anymore as it will install PE1 as best route in this case PE3 and PE1 will not have the second route via PE2
the advertise-best external will let PE2 to keep advertise its route so PE1 and PE3 can use it with PIC functionality
hope this help
