Which MPLS attribute is required or links to carry a given Cisco MPLS TE tunnel?

Next-hop backup tunnel is an FRR protection mechanism for rerouting traffic around failures, not an attribute that links a TE tunnel to specific paths.

Tunnel path-selection metric affects which metric (TE or IGP) CSPF uses to compute the path, but it does not link or constrain which specific links carry the tunnel.

The TE tunnel destination address is a required endpoint configuration parameter, but it is not the attribute that administratively links the tunnel to specific network links.

Affinity is the MPLS TE attribute that controls path selection by specifying which link administrative groups (colors) a tunnel requires or excludes. It is configured on both the tunnel interface and individual links, and the CSPF algorithm uses these bit-mask values to determine which links are eligible to carry the tunnel. Without affinity configuration, the tunnel has no constraint on which links it can use.