3Com 3031 Command Reference Manual page 774

774 C 8: MPLS B
HAPTER ASIC
mpls ldp advertise
C C
ONFIGURATION OMMANDS
[3Com] mpls ldp
Syntax
mpls ldp advertise { implicit-null | explicit-null | non-null }
undo mpls ldp advertise { implicit-null | explicit-null | non-null }
View
System view
Parameter
explicit-null: Specifies to assign explicit null label to the penultimate hop at egress.
implicit-null: Specifies to assign implicit null label to the penultimate hop at egress.
non-nul: Specifies to assign normal label to the penultimate hop at egress.
Label value 0 stands for IPv4 Explicit NULL Label, which is valid only at the
bottom of label stack. That is, the label stack must be popped and forwarded
as IPv4 header.
Label value 1 stands for Router Alert Label, which is valid except at the bottom
of label stack. When receiving messages with label value 1 at the top of the
label stack, the system forwards them into local software module for further
processing. If a lower-layer label is to be forwarded, it must be put with Router
Alert Label.
Label value 2 stands for IPv6 Explicit NULL Label, which is valid only at the
bottom of label stack. That is, the label stack must be popped and forwarded
as IPv4 header.
Label value 3 stands for Implicit NULL Label, which can be distributed and
forwarded, but cannot be placed in encapsulation. When LSR switches
top-layer labels, it only need to pop the labels, but cannot replace them when
using label 3 to replace the original label.
Labels 4~15 are reserved.
Description
Using the mpls label advertise command, you can specify what label is to be
assigned to the penultimate hop at egress node. Using the undo mpls label
advertise command, you can restore the default value.
When the keyword explicit-null is selected, the m-layer label of a packet with
m-layer label parameter will be popped at the penultimate LSR of the LSP, but not
the egress LSR. This can lower operation restriction at egress node and mitigate
the traffic at the egress node to a degree.
By default, implicit label is assigned to the penultimate hop at egress node.
If explicit null label is assigned to the penultimate hop, it can only reside at the
bottom of the label stack.
Example
Specify at the egress to allocate general labels to the penultimate hop.