When Microsoft peering is configured on your ExpressRoute circuit, the Microsoft edge routers establish a pair of BGP sessions with the customer or provider edge routers. In the first place no routes are advertised to a customer network. To enable route advertisements to those networks, a route filter must be associated. A route filter lets identify services a customer want to consume through your ExpressRoute circuit's Microsoft peering. It is essentially a white list of all the BGP community values that should be announced. Once a route filter resource is defined and attached to an ExpressRoute circuit, all prefixes that map to the BGP community values are advertised to the customer network.