A BGP router can send a Route Refresh message to its peer only if both have advertised the route refresh capability (code 2). The Route Refresh message is a request for the peer to re-send all or some of its routes associated with a particular pair of AFI/SAFI values. AFI/SAFI values are the same ones used in the MP-BGP capability (see the section titled Multi-protocol BGP attributes).
7450, 7750, and 7950 routers only send Route Refresh messages for AFI/SAFI associated with VPN routes that carry Route Target extended communities, such as VPN-IPv4, VPN-IPv6, L2-VPN, MVPN-IPv4 and MVPN-IPv6 routes. By default, routes of these types are discarded if, at the time they are received, there is no VPN that imports any of the route targets they carry. If at a later time a VPN is added or reconfigured (in terms of the route targets that it imports), a Route Refresh message is sent to all relevant peers, so that previously discarded routes can be relearned.