In which conditions does LDP use non-null labels

LDP uses non-null labels in any of the following situations:
Non-null labels are configured in the MPLS view. Upon receiving a Label Mapping message, the egress node advertises non-null labels if it does not respond with a Label Release message, regardless of whether the LSP is a liberal LSP.
A transit LSP is established.
On a remote LDP session, the LSP triggering mode is all if no downstream local session exists. The LSR functions as the agent egress node to assign non-null labels to FECs along the local route.

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