Juniper ACX1000 Configuration Manual page 647

Copyright © 2017, Juniper Networks, Inc.
When all of the following conditions are met, the incoming TTL is set to the TTL value
found in the immediate inner header:
The outer label is popped as opposed to being swapped
The TTL processing model is configured to pipe
The inner header is MPLS or IP
If any of those conditions is not met, then the incoming TTL is set to the TTL value found
in the outermost label. In all cases, the TTL values of any further inner labels are ignored.
When an IP packet is exposed after MPLS pops all the labels that should be popped,
MPLS passes the packet to IP for further processing, including TTL checking. When the
uniform tunnel model for TTL processing is in effect, MPLS sets the TTL value of the IP
packet to the incoming TTL value that was just set. In other words, the TTL value is copied
from the outermost label to the IP packet. When the pipe model for TTL processing is in
effect, the TTL value in the IP header is left unchanged.
If an IP packet is not exposed by the label popping, then MPLS performs the TTL validation.
If the incoming TTL is less than 2, the packet is dropped. If innermost packet is IP, an
ICMP packet is built and sent. If the TTL does not expire and the packet needs to be sent
out, the outgoing TTL is determined by the rules for outgoing MPLS packets.
Chapter 20: Configuring MPLS and Pseudowires
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