LSRs only advertise labels for FECs when they have matching forwarding entries for
those FECs??”most commonly IP routes learned using the IGP. LSPs may be established
either using independent control (in which case, an LSR may advertise a label for a
FEC regardless of whether it has received a label mapping from a downstream LSR)
or using ordered control (in which case, all LSRs other than the egress LSR may only
advertise a label for a FEC once they have received a mapping from a downstream
LSR). Ordered control ensures that LSPs are established end to end, whereas with
independent control, partial LSPs may be created; though in a network where all LSRs
share a correctly configured IGP, this condition may only occur temporarily during
routing transients. Independent control is thought to result in slightly faster network
convergence than ordered control.
LDP uses the following label messages to distribute labels:
?– Label Request Optionally sent by an upstream LSR to request a label from its
downstream neighbour.
?– Label Mapping Sent by a downstream LSR to its upstream neighbour to advertise
a label for a FEC. It may be sent either in response to a label request, known
as downstream-on-demand label distribution, or simply when the LSR is ready
to advertise the label, known as downstream-unsolicited label distribution.
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