8
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.
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.
