After configuring VPNv4, a remote PE shows the vpnv4 route but traffic fails; 'show bgp vpnv4 unicast' shows next‑hop is PE1's loopback that is not in the core routing table. What configuration step would fix reachability from other PEs?

The PE's loopback interface is used as the BGP next-hop for VPNv4 routes. For other PEs to reach this next-hop and forward VPN traffic, the loopback address must be reachable through the core network. Advertising the PE loopback into the IGP (like OSPF or EIGRP) running in the core ensures all core routers, including other PEs, have a route to it. Enabling LDP on the loopback is for MPLS label distribution, not IP reachability. Setting the route distinguisher to the loopback address is incorrect; the RD differentiates VPN routes, not provides reachability. Configuring the CE to run the provider IGP is a security and design flaw, as CEs should not participate in the provider's core routing.