By default, the PE performs VPN target filtering on the received IPv4 VPN routes. The routes passing the filter are added to the routing table, and the others are discarded. If the PE is not configured with VPN instance, or the VPN instance is not configured with the VPN target, the PE discards all the received VPN IPv4 routes.
In Inter-AS VPN Option B, you do not need to configure VPN instances on the ASBRs. An ASBR must save all the VPNv4 routes and advertises the VPNv4 routes to the remote ASBR. In this case, the ASBR must accept all the VPNv4 routing information without the VPN target filtering.