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.

Other related questions:
How to view the electronic labels
You can use commands to view the electronic labels. To view the part number and production date, run the display elabel command. [Board Properties] BoardType=CX22EMFF BarCode=21023514201091000095 Item=02351420 Description=S3752P-EI-48S, S3752P-EI-48S Mainframe(48 100 BASE-X ports and 2 100/1000 BASE-X ports and 2 SFP GE (1000 BASE-X) ports (SFP Req.) and DC -48V) Manufactured=2009-01-24 VendorName=Huawei IssueNumber=00 CLEICode= BOM=02351420 BarCode: electronic label Manufactured: production date BOM: part number

What is an implicit-null label
The implicit-null label is label 3, which is used and assigned by an LSR. The implicit-null label, however, is not used in encapsulation. To switch the label, an LSR needs to pop up the label stack upon finding that the new label is an implicit-null label. If the downstream forwarding entry uses label 3, the penultimate hop directly sends packets to the egress node without attaching any label. The egress node follows the IP forwarding process to forward the packets as if the packets contain no MPLS label.

What is an explicit-null label
The explicit-null label is label 0, which takes effect only at the bottom of the stack. An LSR must pop up the label stack and forward the explicit-null label based on the IPv4 header information. The penultimate hop attaches label 0 to the bottom of the stack and forwards the stack to the egress node. The egress node does not search in the ILM if a packet has label 0. It pops the label 0 and determines the forwarding path based on the IP header information (if the label stack has only one layer).

What types of LDP labels are there
LDP labels include: Explicit-null label Implicit-null label Non-null label

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