20
Figure 10 TTL processing when TTL propagation is enabled
•
Disable
TTL
propagation
—When an LSR labels a packet, it does not copy the TTL value of
the original IP packet to the TTL field of the label, and the label's TTL is set to 255. When an
LSR pops the stack-top label, it does not copy the label's TTL to the original packet, and if the
LSR is the egress LSR, it decreases the TTL value of the original packet by 1. Other LSRs do
not change the TTL value of the original packet. In this mode, the tracert result does not show
the hops within the MPLS backbone, as if the ingress and egress were connected directly.
Figure 11 TTL processing when TTL propagation is disabled
Configuration guidelines
As a best practice, configure the same TTL processing mode on all LSRs along an LSP.
To enable IP TTL propagation for a VPN, you must enable it on all PE devices in the VPN, so that you
can get the same traceroute result (hop count) from those PEs. For more information about PEs, see
"Configuring MPLS L3VPN."
Configuration procedure
To configure TTL propagation of MPLS:
Step Command
Remarks
1.
Enter system view.
system-view
N/A
2.
Enter MPLS view.
mpls
N/A
3.
Enable MPLS TTL
propagation.
ttl propagate
{
public
|
vpn
}
Optional.
Enabled only for public network
packets by default.