H3C Technologies H3C S7500E Series Switches - Manual (Page 66)

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Managing and Optimizing MPLS Forwarding
Configuring TTL Processing Mode at Ingress
At the ingress of an LSP, a label stack encapsulated with a TTL field is added to each packet. Whether
the label TTL takes the IP TTL or not depends on whether IP TTL propagation is enabled:
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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 2-10 Label TTL processing when IP TTL propagation is enabled
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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 2-11 Label TTL processing when IP TTL propagation is disabled
Follow these steps to configure IP TTL propagation of MPLS: