Specifying the label type advertised by egress
In an MPLS network, an egress node can advertise the following types of labels:
Implicit null label with a value of 3.
Explicit null label with a value of 0.
Non-null label. The value range for a non-null label is 16 to 1048575.
For LSPs established by a label distribution protocol, the label advertised by the egress node determines how the penultimate hop processes a labeled packet.
If the egress node advertises an implicit null label, the penultimate hop directly pops the top label of a matching packet.
If the egress node advertises an explicit null label, the penultimate hop swaps the top label value of a matching packet with the explicit null label.
If the egress node advertises a non-null label, the penultimate hop swaps the top label of a matching packet with the label assigned by the egress node.
Configuration guidelines
As a best practice, configure the egress node to advertise an implicit null label to the penultimate hop if the penultimate hop supports PHP. If you want to simplify packet forwarding on egress but keep labels to determine QoS policies, configure the egress node to advertise an explicit null label to the penultimate hop. Use non-null labels only in particular scenarios. For example, when OAM is configured on the egress node, the egress node can get the OAM function entity status only through non-null labels.
As a penultimate hop, the device accepts the implicit null label, explicit null label, or normal label advertised by the egress node.
For LDP LSPs, the mpls label advertise command triggers LDP to delete the LSPs established before the command is executed and re-establishes new LSPs.
For BGP LSPs, the mpls label advertise command takes effect only on the BGP LSPs established after the command is executed. To apply the new setting to BGP LSPs established before the command is executed, delete the routes corresponding to the BGP LSPs, and then redistribute the routes.
Configuration procedure
To specify the type of label that the egress node will advertise to the penultimate hop:
Step | Command | Remarks |
|---|---|---|
1. Enter system view. | system-view | N/A |
2. Specify the label type advertised by the egress node to the penultimate hop. | mpls label advertise { explicit-null | implicit-null | non-null } | By default, an egress node advertises an implicit null label to the penultimate hop. |