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Rules for Processing on an LSR
On an LSR—where an MPLS packet is label-switched
after processing on the line module—the TTL value in the swapped-to
label is decremented by 1 from the incoming TTL value when the swapped-to
label is not implicit-null. When the swapped-to label is implicit-null
(for example, in a PHP configuration), the inner or exposed header's
TTL is either left unchanged (when the forwarded option for the mpls ip propagate-ttl command
has been configured) or is decremented by 1 from the incoming TTL
value. If MPLS needs to push more labels, it sets the TTL for each
label according to the following LER rules, because for those labels
the router effectively is an ingress LER.
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