7
• 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
When you specify the label type advertised by egress, follow these restrictions and guidelines:
• If the penultimate hop supports PHP, configure the egress to advertise an implicit null label to
the penultimate hop as a best practice.
• If you want to simplify packet forwarding on the 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, 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:
1. Enter system view.
N/A
2.
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.
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