Juniper JUNOSE, Configuration Manual

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the stale mappings from the LSR. The configurable value represents the maximum
time that the neighbor waits for the restarting LSR to reestablish the LDP session.
This enables the neighbor to avoid having to wait an unreasonably long time set by
the reconnect timeout value from the restarting LSR.
If the recovery time value in the FT session TLV is zero when a neighbor receives
the new LDP initialization message, the neighbor deletes all the stale mappings from
the LSR.
If the recovery time value is nonzero, the neighbor starts a neighbor recovery timer
set to the lesser of the two values, the recovery time value and its own configurable
maximum recovery timeout value. The neighbor also cancels its neighbor liveness
timer because the LDP session has been reestablished; it is now waiting on the
successful completion of the restart.
The restarting LSR and its neighbors then exchange label mapping information. When
a neighbor receives a label–to–FEC binding that matches a stale entry, it removes
the staleness marker from the entry. If instead the neighbor receives a new label for
the same FEC that is in a stale entry, the neighbor updates the entry with the new
label and removes the staleness marker from the entry.
The neighbor deletes any stale entries that remain when the neighbor recovery timer
expires.
Dynamic exchange of the graceful restart capability is not supported. In some
circumstances, such as when a standby SRP module is removed, an LSR that has
communicated to neighbors that it supports graceful restart might subsequently be
unable to do so. In such cases, the neighbors receive no indication of that change in
support unless you bounce the LDP sessions, for example by issuing the
clear mpls
ldp neighbor command.
LDP-IGP Synchronization
LDP is often used to establish MPLS LSPs throughout a complete network domain
using an IGP such as OSPFv2 or IS-IS. In such a network, all links in the domain have
IGP adjacencies as well as LDP adjacencies. LDP establishes the LSPs on the shortest
path to a destination as determined by IP forwarding.
MPLS data packets can be discarded in these networks when the network IGP is
operational on a link for which LDP is not fully operational, because there is no
coupling between the LDP operational state and the IGP. When LDP is not fully
operational, LDP is considered to not be synchronized with the IGP.
This issue is especially significant for applications such as a core network that does
not employ BGP. Another example is an MPLS VPN where each given PE router
depends on the availability of a complete MPLS forwarding path to the other PE
routers for each VPN that it serves. This means that along the shortest path between
the PE routers, each link must have an operational hello adjacency and an operational
LDP session, and MPLS label bindings must have been exchanged over each session.
When LDP has not completed exchanging label bindings with an IGP next hop, traffic
is discarded if the head end of the LSP forwards traffic because the LSP is assumed
to be in place. The following are some examples of when this can happen.
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LDP-IGP Synchronization
JUNOSe 11.0.x BGP and MPLS Configuration Guide