The ttl propagate command sets the TTL propagate mode of MPLS packets to uniform.
The undo ttl propagate command sets the TTL propagate mode of MPLS packets to pipe.
By default, the TTL propagate function is enabled and the MPLS TTL processing mode is uniform.
Usage Scenario
Uniform
When IP packets reach the ingress node of an MPLS network, the IP TTL decreases by one and is mapped to the MPLS TTL field. In this manner, packets are processed in the standard mode used on the MPLS network. On the egress node, the MPLS TTL decreases by one and is mapped to the IP TTL field. The traceroute output shows the path that the packets pass by.
Pipe
On an MPLS network, the IP TTL does not decrease by one at each hop. The traceroute output hides all the hops on the MPLS backbone network, as if the ingress node is directly connected to the egress node.
In MPLS VPN applications, the MPLS backbone network needs to be hidden to ensure network security. The pipe mode is recommended for private network packets.
Precautions
The ttl propagate command only take effect on LSPs that are to be set up. Before using the function on LSPs that have been set up, run the reset mpls ldp command to reestablish the LSPs.