Specifying the label type advertised by the egress
In an MPLS network, an egress 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 determines how the penultimate hop processes a labeled packet.
If the egress advertises an implicit null label, the penultimate hop directly pops the top label of a matching packet.
If the egress 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 advertises a non-null label (normal label), the penultimate hop swaps the top label of a matching packet with the specific label assigned by the egress.
Configuration guidelines
If the penultimate hop supports PHP, HP recommends that you configure the egress to advertise an implicit null label to the penultimate hop. If you want to simplify packet forwarding on the egress but keep labels in packets for the egress to determine QoS policies, you can configure the egress to advertise an explicit null label to the penultimate hop. HP recommends that you do not use non-null labels except in some special scenarios. For example, when OAM is configured on the egress, the egress 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 device.
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 for 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 to the penultimate hop. | mpls label advertise { explicit-null | implicit-null | non-null } | By default, an egress advertises an implicit null label to the penultimate hop. |