H3C Technologies H3C S12500-X Series Switches - Manual (Page 19)

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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:
• H3C recommends setting 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, so that you
can get the same traceroute result (hop count) from those PEs.
To enable TTL propagation:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enable TTL propagation. mpls ttl propagate { public |
vpn }
By default, TTL propagation is enabled only
for public-network 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.