The
label mapping message identifies both the FEC and the label to be advertised for
that FEC. Both the FEC and label are encoded using TLV (Type-Length-Value)
syntax??”as are all LDP objects.
?– Label Withdraw Sent by a downstream LSR to its upstream neighbour to invalidate
a previously advertised label mapping.
426 Chapter 14
?– Label Release Sent by an upstream LSR to its downstream neighbour to indicate
that it will no longer retain a label mapping. It may be sent either in response
to a Label Withdraw or because the upstream LSR is using conservative label retention
(i.e., it will only retain a label if it was learned from the next hop for that
FEC in the IP routing table) and either the downstream LSR has advertised a FEC
when it is not the next hop for that FEC, or the downstream LSR is no longer the
next hop for the FEC. It is more common, however, to use liberal label retention,
where the LSR retains all label mappings learned for a FEC and simply uses the
one corresponding to the current next hop. Liberal label retention results in faster
convergence since, when the next hop changes, the LSR already has a label for that
next hop, but of course requires the LSR to store more label mappings.
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