H3C Technologies H3C SR8800 - Manual (Page 33)

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• With IP TTL propagation enabled: When the ingress labels a packet, it copies the TTL value of the
original IP packet to the TTL field of the label. When an LSR forwards the labeled packet, it
decrements the TTL value of the label at the stack top by 1. When an LSR pops a label, it copies the
TTL value of the label at the stack top back to the TTL field of the IP packet. In this case, the TTL value
of a packet is decreased hop by hop when forwarded along the LSP. Therefore, the result of tracert
will reflect the real path along which the packet has traveled.
Figure 10 Label TTL processing when IP TTL propagation is enabled
• With IP TTL propagation disabled: When the ingress 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 TTL is set to 255. When an LSR
forwards the labeled packet, it decrements the TTL value of the label at the stack top by 1. When an
LSR pops a label, it compares the IP TTL and the label TTL and uses the smaller value as the TTL of
the IP packet. In this case, the result of tracert does not show the hops within the MPLS backbone,
as if the ingress and egress were connected directly.
Figure 11 Label TTL processing when IP TTL propagation is disabled
To configure IP 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 IP TTL
propagation.
ttl propagate { public | vpn }
Optional.
Enabled for only public network
packets by default.