Juniper JUNOSE, Configuration Manual

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TTL Processing on Incoming MPLS Packets
The flow chart on Figure 51 on page 215 illustrates TTL processing on incoming MPLS
packets. On a transit LSR or an egress LER, MPLS pops one or more labels and can
push one or more labels. The incoming TTL of the packet is determined by the
configured TTL processing tunnel model.
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.
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MPLS Label Switching and Packet Forwarding
JUNOSe 11.0.x BGP and MPLS Configuration Guide