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MPLS TTL Propagation Flexibility for LDP-signaled LSPs

We support disabling time-to-live (TTL) propagation at a more granular level. You can disable TTL propagation specifically for LDP-signaled label-switched paths (LSPs). When a route is very long, disable TTL propagation to ensure that the TTL doesn't expire while the packet is traversing the path. This feature also gives you more flexibility in hiding your network topology.

To disable TTL propagation for LDP-signaled LSPs, use the no-propagate-ttl statement at the [edit protocol ldp] hierarchy level.

Note:

If the TTL value of the top label is less than the TTL value of the bottom label at an egress node, Junos OS copies the TTL value from the top label to the bottom label. In this case, the TTL value can still propagate down even when no-propagate-ttl is configured.
Note: If you don't configure the no-propagate-ttl statement, the device copies the MPLS TTL value to the inner IP header during the MPLS label removal process. The device copies that value only when the TTL of the inner IP header is greater than or equal to the MPLS TTL value. If the TTL of the inner IP header is less than the MPLS TTL value, the device does not copy the MPLS TTL value.

[See no-propagate-ttl.]

Purpose

The purpose is to support the configuration and functionality of the no-propagate-ttl option at the [edit protocol ldp] hierarchy.

LDP No Propagate TTL Configuration

The following command results in LDP following the no propagate TTL behavior.

If you configure the global no-propagate-ttl option with this command, there is no change in behavior.

The no-propagate-ttl option has no impact on the independent RSVP LSP. However, when you use this option with LDPoRSVP, the TTL action of the RSVP LSP decides the TTL propagation behavior.

This means that when the router is acting as an ingress node for LDP and RSVP for the LDPoRSVP LSP, if you configure no-decrement-ttl for the RSVP LSP, then the LDPoRSVP does not propagate TTL. And if you don't configure the no-decrement-ttl option for the RSVP LSP, then it propagates TTL.

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