HP FlexNetwork 10500 Series Configuration Manual page 18

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
TTL 252
TTL 253 TTL 252
LSR A
Ingress
TTL
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
TTL 255
TTL 3 TTL 2
LSR A
Ingress
Follow these guidelines when you configure TTL propagation:
•
As a best practice, set the same TTL processing mode on all LSRs of an LSP.
•
To enable TTL propagation for a VPN, you must enable it on all PE devices in the VPN. Then,
you can get the same traceroute result (hop count) from those PEs.
To enable TTL propagation:
Step
1. Enter system view.
2. Enable TTL
propagation.
TTL 251
TTL 252
LSR B LSR C
IP TTL TTL
Copy the TTL value
TTL 254
TTL 2
LSR B LSR C
TTL IP TTL
Command
system-view
mpls ttl propagate { public |
vpn }
9
TTL 250
TTL 252 TTL 249
LSR D
TTL 249
TTL 249
Label TTL
Copy the label TTL value to the IP
packet and pops the label at the
penultimate hop
TTL 253
TTL 2 TTL 2
LSR D
TTL Label TTL
Remarks
N/A
By default, TTL propagation is enabled for
public network packets and is disabled for
VPN packets.
This command affects only the propagation
between IP TTL and label TTL. Within an
MPLS network, TTL is always copied
between the labels of an MPLS packet.
TTL 248
LSR E
Egress
TTL 1
LSR E
Egress