8
324BConfiguration 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
node to the penultimate hop.
mpls label advertise
{
explicit-null
|
implicit-null
|
non-null
}
By default, an egress node
advertises an implicit null label to
the penultimate hop.
19B
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. 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.
Figure 6 TTL propagation
When TTL propagation is disabled, the ingress node sets the label TTL to 255. Each LSR on the LSP
decreases the label TTL value by 1. The LSR that pops the label does not change the IP TTL value
when popping the label. Therefore, the MPLS backbone nodes are invisible to user networks, and
the IP tracert facility cannot show the real path in the MPLS network.
Figure 7 Without TTL propagation
Follow these guidelines when you configure TTL propagation:
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