H3C Technologies H3C S12500-X Series Switches - Manual (Page 18)

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• 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, H3C 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. H3C 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.
Configuring TTL propagation
When TTL propagation is enabled, the ingress node copies the TTL value of an IP packet to the TTL field
of the label. Each LSR on the LSP decreases the label TTL value by 1. The LSR that pops the label copies
the remaining label TTL value back to the IP TTL of the packet, so the IP TTL value can reflect how many
hops the packet has traversed in the MPLS network. The IP tracert facility can show the real path along
which the packet has traveled.