Command Reference Guide

3Com Router 3000 Ethernet Family
Command Reference Guide Chapter 1 MPLS Basic Configuration Commands
3Com Corporation
1-19
Parameter
explicit-null: Specifies at egress to distribute an explicit null label to the penultimate
hop.
implicit-null: Specifies at egress to distribute an implicit null label to the penultimate
hop.
non-null: Specifies at egress to distribute a normal label to the penultimate hop.
 Note:
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 the 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 the IPv6 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
needs to pop the labels, but cannot replace them when using label 3 to replace the
original label.
Labels 4 to 15 are reserved.
Description
Use the mpls label advertise command to specify at the egress the type of the label to
be distributed to the penultimate hop.
Use the undo mpls label advertise command to restore the default.
When the keyword explicit-null is selected, the m-layer label of a packet with m-layer
label parameter is popped at the penultimate LSR of the LSP, instead of at the egress
LSR. This can lower the stack operation demands at egress node and somewhat
mitigate the load on the egress node.
By default, the implicit label is assigned to the penultimate hop.
If the 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 distribute a normal label to the penultimate hop.
[3Com -mpls] mpls label advertise non-null