H3C Technologies H3C S7500E Series Switches - Manual (Page 67)

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To do… Use the command… Remarks
Enter system view system-view —
Enter MPLS view mpls —
Enable MPLS IP TTL propagation ttl propagate { public | vpn }
Optional
Enabled for only public network
packets by default
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Within an MPLS domain, TTL is always copied between multiple levels of labels. The ttl
propagate command affects only the propagation of the IP TTL to the TTL of an MPLS label.
Therefore, this command takes effect only when it is configured on the ingress.
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When a local packet is transmitted, the IP TTL value of the packet is always copied regardless of
whether IP TTL propagation is enabled or not. This ensures that the local administrator can
tracert for network diagnoses.
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If you enable MPLS IP TTL propagation for VPN packets on one LSR, you are recommended to
do so on all related PEs as well, so that you can get the same result when tracerting from those
PEs.
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Some VPN packets carry two layers of labels, outer and inner, for transmission in the public
network and private network respectively. The LSQ1SRP1CB engine and EA series LPUs have to
copy the IP TTL of private network packets to the inner label before they can copy the IP TTL to
the outer label. However, SD series and EB series can copy the IP TTL of private packets to the
outer label directly.
Configuring Sending of MPLS TTL Timeout Messages
After sending of MPLS TTL timeout message is enabled on an LSR, when the LSR receives an MPLS
packet that carries a label with TTL being 1, it will generate an ICMP time exceeded message, and
send the message to the packet sender in one of the following ways:
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If there is a route from the LSR to the packet sender, the LSR sends the TTL timeout message to
the packet sender directly through the IP route.
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If there is no route from the LSR to the packet sender, the LSR forwards the TTL timeout
message along the LSP to the egress, which will send the TTL timeout message to the packet
sender.
Usually, for an MPLS packet carrying only one level of label, the first method is used; for an MPLS
packet carrying a multi-level label stack, the second method is used. However, as autonomous system
border routers (ASBRs), superstratum PEs or service provider-end PEs (SPEs) in Hierarchy of VPN
(HoVPN) applications, and carrier backbone PEs in nested VPNs, may receive MPLS VPN packets
that carry only one level of labels but there are no IP routes from these devices to the packet senders,