How to configure the egress node to assign explicit-null labels to the penultimate hop

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If the label of the penultimate hop is implicit-null, the value of the label that the egress node assigns to the penultimate hop is 3. If the label of the penultimate hop is non-null, the PHP feature is not used and the egress node properly assigns labels to the penultimate hop. The value of the label is equal to or larger than 16.

Typical failure: The label distribution mode is not modified, or the label distribution mode is set to non-null.

Solution: In the MPLS view, set the label distribution mode to explicit-null.

Other related questions:
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).

How to determine the type of label allocated on the penultimate hop
By default, the egress node assigns implicit-null labels to the penultimate hop. When an implicit-null label is assigned to an LSR, the LSR directly pops the labels without having to replace the label at the top of the stack with this implicit-null label. If the penultimate hop adds a label to the label stack, the label distribution mode has been changed. Typical failure: In the MPLS view, the label assigned to the penultimate hop is explicit-null or non-null. The penultimate hop is assigned a label other than 3. You can check the label distribution mode according to the configuration in the MPLS view. Solution: Configure the PHP feature in the MPLS view and set the label distribution mode to implicit-null. Or you can delete the configuration of the label distribution mode.

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.

Do I need to specify the next hop or egress when configuring a static LSP on the ingress node
When configuring a static LSP on the ingress node, ensure that configuration items match routing information, including the destination address, mask, next hop, and outbound interface;otherwise,the LSP cannot become Up. The system calculates the next hop if you specify the outbound interface. The calculated next hop is the IP address of the local outbound interface. The system calculates the outbound interface if you specify the next hop (the inbound interface of the peer end). The calculated outbound interface is still the local outbound interface. Therefore, errors may occur during the static LSP configuration on the ingress node. Pay attention to the following points: If the outbound interface is specified for a static route, you must specify the outbound interface when configuring the static LSP on the ingress node. In other situations, you need to specify the next hop rather than only the outbound interface. The fundamental rule is that the static ingress LSP route fully matches routing information.

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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