A service provider has configured an MPLS TE tunnel from PE1 (headend) to PE3 (tailend) that traverses a lower-utilization path across the core. After bringing the tunnel up, the operator confirms it is signaled and operational, but a traceroute to prefixes reachable via PE3 shows traffic is still following the native IGP shortest path and not entering the tunnel. Which configuration on PE1 will cause the IGP to consider the TE tunnel as a valid path for reaching destinations downstream of the tailend?