Configuring a TTL processing mode for an LSR
An LSR can process the TTL field in the either of the following ways:
Enable TTL propagation—When an LSR labels a packet, it copies the TTL value of the original
packet (the IP TTL value of an IP packet or the TTL value in the top label of a labeled packet) to the
TTL field of the newly added label. When an LSR pops the stack-top label, it copies the TTL value of
the stack-top label to the TTL field of the original packet. In this mode, the TTL value of a packet is
decreased hop by hop when forwarded along the LSP. The tracert result reflects the real path along
which the packet has traveled.
Figure 21 TTL processing when TTL propagation is enabled
Disable TTL propagation—When an LSR 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's TTL is set to 255. When an LSR pops
the stack-top label, it does not copy the label's TTL to the original packet, and if the LSR is the egress
LSR, it decreases the TTL value of the original packet by 1. Other LSRs do not change the TTL value
of the original packet. In this mode, the tracert result does not show the hops within the MPLS
backbone, as if the ingress and egress were connected directly.
Figure 22 TTL processing when TTL propagation is disabled
HP recommends configuring the same TTL processing mode on all LSRs along an LSP.
To enable IP 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. For more information about PEs, see