This specifies whether this co-routed tunnel imposes a fixed TTL on the MPLS label or inherits the TTL from the encapsulated frame, applying identically to both the forward and reverse legs since they travel the same physical route. An admin sets this to fixed on provider-facing co-routed circuits for the same topology-hiding reason as on unidirectional tunnels, hiding internal core hop counts from customer traceroutes in both directions at once. For example, a fixed TTL policy here means a customer's traceroute in either direction across this circuit shows no visible intermediate hops.